Three skin whitening assays combining the semiconductor technology was demonstrated for screening the drug candidate and discussing the mechanisms of melanogenesis in medical cosmetics. We proposed a miniaturized photometric system using complementary metal oxide semiconductor (CMOS) phototransistor as a detector with high sensitivity. The CMOS phototransistor was a small, portable optoelectronic device with enlarged depletion region of the outer N well/Pwell ring-shaped photodiode, able to cause a sensitive shift of the threshold voltage and enhance photocurrent response. The device was further connected to IC system to show it ability for high-throughput screening needed in the pharmaceutical industry. Three methods for evaluating effects of skin whitening agent were applied, including 3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyltetrazoliu m bromide (MTT) assay, melanin amount measurement, and tyrosinase assays. The change of light absorbance due to enzymeatic catalysis was easily measured with the proposed system. The skin whitening effects by kojic acid was determined and comparable to the commercial system.