Abstract: An effective algorithm for detecting pupil size based on image processing was established. Three features reflecting differences between opiate addicts and normal subjects in pupillary light reflex were extracted by applying this algorithm, including absolute amplitude of contraction (AAC), relative amplitude of contraction (RAC) and slope of contraction velocity (SCV). Effects of addiction, sex, myopia, aging, sleep deprivation on the three features were studied. None of three features was found significant differences between sexes, myopia, sleep deprived and normal subjects. Only RAC increased significantly in aging subjects in comparison to opiate addicts, The features of pupillary light reflex in opiate addicts decreased significantly not only comparing with normal subjects, but the subjects with potential facts that may alter their pattern of pupil light reflex as well. Our study provided basic data for future establishment of a rapid, non-contact method in distinguishing opiate addicts from other subjects based on pupillary light reflex.