The organ of Corti, the peripheral receptor of auditory sense in mammals, is composed of some sensory and supporting cells derived from surface ectoderm. It is well known that formation and maturation of sensory and supporting cells of organ of Corti in human is completed at fetal period but in rodents such as hamster, rat or mouse maturation of organ of Corti occurs after birth. In this experiment the author studied the chronological and morphological accepts of differentiation of neuroepithelial cells, sensory and supporting cells constituting organ of Corti in albino rats. Fetal rats of 17th and 21st day of gestation and rats of 3rd day, 7th day, 14th day and 21st day after birth were decapitated and cochlear specimens were obtained with microdissection technique. All the specimens were prefixed in 2% glutaraldehyde-2.5% paraformaldehyde for 2 hours, decalcified in for 14 days, post fixed in 1% osmium tetraoxide for 2 hours, dehydrated and embedded in Epon Araldite 502. Ultrathin section and double stain with uranyl acetate and lead citrate were performed and organ of Corti in middle cochlear turn was observed with electron microscope. The results obtained were as follows ; 1) Primordial cells of organ of Corti with euchromatin predominant oval nuclei and many vacuoles in cytoplasm were observed in fetal rats of 17th and 21st day and sensory and supporting cells, were distinguished from 3rd day after birth. 2) Intercellular spaces between hair cells were formed from 7th day after birth and completed at 14 day of the birth, culticular plate were observed from 7 day after birth and cell junctions from 21st day of fetal life. 3) In the sensory cells of organ of Corti kinocilium was observed at 21st day of fetal life and stereocilia were observed in inner hair cell and 1st row outer hair cell at 7th day after birth and in all inner and outer hair cells at 14th day after birth. 4) Mitochondriae in sensory cells were observed in inner hair cell at 14th day after birth and in subcuticular, subnuclear region and near the plasma membrane of all inner and outer hair cells at 21st day after birth. 5) Tonofilaments in inner pillar cells were observed from 7 day after birth as microtublue bundles of 1μm in length and extended from basement membrane to reticular membrane at 21st day after birth. 6) Tectorial membrane was observed from 7th day after birth as evenly distributed microfilaments and inferior portion of tectorial membrane with electron dense microfilament bundle was observed at 21st day after birth. It is consequently suggested that organ of Corti of fetal rat is composed of primordial cell at fetal life and kinocilium is observed at 21st day of fetal life and stereocilia are formed from medial side of organ of Corti at 7th day after birth and morphological maturation of hair cell, intercellular space, inner pillar cell and tectorial membrane is completed from 14th day to 21st day after birth in albino rats.