I have developing a wordpress theme. But I have problem in post thumbnail to set featured image. When I set a featured image with 300x210 px then it become 221x155. In functions.php i added this code. Is it an issue with css file? I need this urgent basis.

    //add support for featured images 
      set_post_thumbnail_size(300, 210);

Into your loop or any theme template file on that note:

the_post_thumbnail();                  // without parameter -> Thumbnail

the_post_thumbnail('thumbnail');       // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium');          // Medium resolution (default 300px x 300px max)
the_post_thumbnail('large');           // Large resolution (default 640px x 640px max)
the_post_thumbnail('full');            // Original image resolution (unmodified)

the_post_thumbnail( array(100,100) );  // Other resolutions

The last one will probably help you. Also, nothing on DaniWeb is urgent :)