In this paper we discuss the feature selection in screening cervical cells by meang of optical power spectra. After analyzing morphological characteristics of normal cells and cancer cells and features of corresponding spectra, we have found the key for recognition. This feature alone can give a correct-recongition rate up to 94.5%, higher than the rate of 92% obtained before by using six features in a similar experiment[1]. We also present the relation between the correct-recognition rate and the dimension of feature space. The results indicate a fall-down of the correct-rate when the feature number Is increased improperly.