Differentiation and degeneration of spikelets in paddy rice has been studied in high yielding Indica×Japonica hybrid cultivar, Milyang 23 and a Japonica type cultivar, Koshihikari. Germinated seeds planted in 5000^-1 a pots filled with submerged soil and cultured under natural conditions. The young panicles of main stem were continuously dissected and observered by Cryo-SEM from the panicle initiation stage, and investigated about formation position of the differentiation and degeneration spikelet within a panicle of 7 days after heading. The degeneration of spikelet appeared simultaneously throughout panicle just after closure of spikelet by the palea and lemma. Differentiated and degenerated spikelets per panicle were about 240, 80 for Milyang 23 and 87, 6 for Koshihikari, respectively. The spikelets degeneration in Milyang 23 was mainly on the secondary and tertiary branch which were developed from primary branch of middle-basal panicle node and hardly not the spikelets of primary branch, and degeneration rate of secondary and tertiary rachis branch and spikelets for Milyang 23 were 2.5 times greater than those of Koshihikari. The proper relation equation between total differentiation or normal spikelets number per panicle(Y) and each rachis branch number were different between cultivars, i.e., Y=5.5X₁+3.0X₂ for Koshihikari as previously proposed, but those of Milyang 23, Y=5.7X₁+3.5X₂+2.8X₃ for total differentiation spikelets and Y=5.6X₁+3.2X₂+2.4X₃ for normally developed spikelets, where X₁, X₂, X₃ are number of primary, secondary, tertiary rachis branch, respectively.