The crazing behavior of polystyrene (PS) was investigated in ethanol for examining failure criteria. The pure shear and uniaxial tension were applied to PS plate specimen in ethanol at room temperature. Only crazing was identified on the specimen surface under both stress states. Crazes were formed at lower stress in ethanol than in air. This was supposed to be caused by plasticization effect of ethanol. For confirming the plasticization effect, differential scanning calorimetry (DSC) was conducted. The glass transition temperature of the ethanol-absorbed PS specimen was greatly lowered to about 40℃. The newly proposed criterion gave the theoretical critical stress ratio being almost consistent with experiment.