This is a contest for amateur writers only. (See rules below.) So, if you have published a book, or more than three stories in professional publications (at professional rates), you are not eligible.
You can submit an unpublished sci-fi short story or novella (up to 17,000 words). The contest is held four times a year, so there are ample chances to win.
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From the website:
L. Ron Hubbard’s Writers of the Future Contest is an opportunity for new and amateur writers of new short stories or novelettes of science fiction or fantasy. No entry fee is required. Entrants retain all publication rights. All awards are adjudicated by professional writers only. Prizes every three months: $1,000, $750, $500, Annual Grand Prize: $5,000 additional!
Rules
1. No entry fee is required, and all rights in the story remain the property of the author. All types of science fiction, fantasy and dark fantasy are welcome.
2. By submitting to the Contest, the entrant agrees to abide by all Contest rules.
3. All entries must be original works, in English. Plagiarism, which includes the use of third-party poetry, song lyrics, characters or another person’s universe, without written permission, will result in disqualification. Excessive violence or sex, determined by the judges, will result in disqualification. Entries may not have been previously published in professional media.
4. To be eligible, entries must be works of prose, up to 17,000 words in length. We regret we cannot consider poetry, or works intended for children.
5. The Contest is open only to those who have not professionally published a novel or short novel, or more than one novelette, or more than three short stories, in any medium. Professional publication is deemed to be payment of at least six cents per word, and at least 5,000 copies, or 5,000 hits.
6. Entries submitted in hard copy must be typewritten or a computer printout in black ink on white paper, printed only on the front of the paper, double-spaced, with numbered pages. All other formats will be disqualified. Each entry must have a cover page with the title of the work, the author’s legal name, a pen name if applicable, address, telephone number, e-mail address and an approximate word count. Every subsequent page must carry the title and a page number, but the author’s name must be deleted to facilitate fair, anonymous judging.
Entries submitted electronically must be double-spaced and must include the title and page number on each page, but not the author’s name. Electronic submissions will separately include the author’s legal name, pen name if applicable, address, telephone number, e-mail address and approximate word count.
7. Manuscripts will be returned after judging only if the author has provided return postage on a self-addressed envelope.
8. We accept only entries that do not require a delivery signature for us to receive them.
9. There shall be three cash prizes in each quarter: a First Prize of $1,000, a Second Prize of $750, and a Third Prize of $500, in US dollars. In addition, at the end of the year the winners will have their entries rejudged, and a Grand Prize winner shall be determined and receive an additional $5,000. All winners will also receive trophies.
10. The Contest has four quarters, beginning on October 1, January 1, April 1 and July 1. The year will end on September 30. To be eligible for judging in its quarter, an entry must be postmarked or received electronically no later than midnight on the last day of the quarter. Late entries will be included in the following quarter and the Contest Administration will so notify the entrant.
11. Each entrant may submit only one manuscript per quarter. Winners are ineligible to make further entries in the Contest.
12. All entries for each quarter are final. No revisions are accepted.
13. Entries will be judged by professional authors. The decisions of the judges are entirely their own, and are final.
14. Winners in each quarter will be individually notified of the results by phone, mail or e-mail.
15. This Contest is void where prohibited by law.