WordNet (r) 3.0 (2006):

straight face
    n 1: a serious facial expression giving no evidence of interest
         or amusement

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "straight face":
   anesthesia, autism, catatonia, chill, chilliness, cold blood,
   cold heart, coldheartedness, coldness, coolness, deadpan,
   decorousness, demureness, dispassion, dispassionateness, dullness,
   earnestness, emotional deadness, emotionlessness,
   expressionlessness, formality, frigidity, frostiness, gravity,
   grimness, heartlessness, iciness, immovability, impassibility,
   impassiveness, impassivity, inexcitability, inexpressiveness,
   lack of affect, lack of feeling, lack of touch, long face,
   objectivity, obtuseness, passionlessness, poker face, sedateness,
   self-absorption, seriousness, sober-mindedness, soberness,
   sobersidedness, sobersides, sobriety, solemnity, solemnness,
   somberness, soullessness, spiritlessness, staidness,
   thoughtfulness, unemotionalism, unexcitability, unexpressiveness,
   unfeeling, unfeelingness, unimpressibility, unimpressionableness,
   unpassionateness, unresponsiveness, unsusceptibility,
   unsympatheticness, untouchability, weightiness, withdrawal