WordNet (r) 3.0 (2006):

unsmiling
    adj 1: not smiling

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "unsmiling":
   cheerless, decorous, demure, discontented, earnest, formal,
   frowning, grave, grim, grim-faced, grim-visaged, humorless,
   in bad humor, infestive, joyless, long-faced, mirthless, miserable,
   out of humor, out of sorts, pleasureless, sedate, serious, sober,
   sober-minded, sobersided, solemn, somber, sorry, sorryish, staid,
   stone-faced, straight-faced, thoughtful, uncheerful, uncheery,
   unhappy, unjoyful, unmirthful, weighty, wretched