Well,

Just like Spiderman comic was made a movie, X-Men comic was made a movie, Fantastic-4 Comic made in to a movie...and many more Comics being made in to movies.

They decided to take the 300 comic and make it in to a Movie.


300 was initially published as a monthly five-issue comic book series by Dark Horse Comics, the first issue published in May 1998. The issues were titled Honor, Duty, Glory, Combat and Victory. The series won three Eisner Awards in 1999: "Best Limited Series", "Best Writer/Artist" for Frank Miller and "Best Colorist" for Lynn Varley. The work was collected as a hardcover graphic novel in 1999.

What we see in the Movie, has been shown for quite some time in the Comic Book.