The pathologist faced with the necessity for examining the placental tissue with a wide variety of gestational ages needs certain criteria for the determination of approximate gestational period in normal placentas. This knowledge enables one to be aware of abnormalities which could be physiological in certain period of gestation. Authors conducted a morphological study on 166 normal human placentas ranging from 6weeds to 44 weeks. These placentas were classifies into 6 groups according to gestationalperiod with the interval of 6 weeks. Gross and microscopic observations were made and tabulated. They are summarized as follows. 1. Gross findings of the developing placenta were characterized by cotyledon formation at the end of the 7th week, subchorionic fibrin deposit, fibrosis, tesselation and Nitatuch layer, all of them appearing by 27 weeks gestation. 2. Microscopical characteristics consisted of endarteritis obliterans of the arterioles by 28 weeks, and intervillous fibrin deposit and thrombosis by 19 weeks of gestational wege. 3. Trophoblastic cells of the terminal villi stowed loBs of con沐(목)nuation after 13 weeks and eventually syncytial trophoblasts remain until full term. Syncytial knots were seen after 20 weeks. Villous stroma changed from myxoid to fibrous nature by 19 weeks gestation. 4. Degenerative changes of the decidua were prominent by 28 weeks gestation, and recomprised of decidual necrosis, fibrin deposit and venous thrombosis. In conclusion, sequential morphological changes of the developing human placentas could enable us to determine the approximate gestational age for unknown placental samples.