From: SciFi.com :

It's official: UPN has announced that it will cancel its teen-alien series Roswell. The network will air the series finale at 9 p.m. ET/PT on May 14. "I would like to thank 20th Century Fox Television, UPN and especially our fans, whose combined efforts helped get us a third season for Roswell," series co-creator Jason Katims said in a statement. "[Executive producer] Ronald D. Moore and I had the fans very much on our minds when we were writing this final episode. We felt they deserved a great ending."

Roswell's fate has been in doubt all season as the series' ratings have dropped against stiff competition from The WB's Superman series, Smallville. The show wrapped production on the current third season in March. UPN took over Roswell this year, after The WB dropped it after two seasons. Roswell has never been a strong ratings contender, but The WB renewed it after its first year as the result of a vigorous campaign by fans that included the mailing of thousands of tiny bottles of Tabasco, the teen aliens' condiment of choice.

The SCI FI Channel will begin airing reruns of all three seasons of Roswell starting this fall.

From TV Guide.com:

ALIENS, GO HOME: After three seasons on the air that's two on the WB and just one on new network UPN Roswell has been cancelled. The steamy teen sci-fi sudser promises to blast off for good with a "shocking and heartrending" series finale on May 14.

From  Zap2it:

The final four episodes of "Roswell" will begin airing on Tuesday, April 23 at 8 p.m. ET and promises have been made that they will resolve the many outstanding storylines, as well as offering a few plot twists along the way.