A study was conducted to clarify the process of egg development and the variation of egg size in the Japanese mitten crab Eriocheir japonicus. The duration of egg development varied from 15 days (oviposition in October) to 79 days (oviposition in January). Hatching was not synchronized with the lunar rhythm. The mean egg diameter in each batch varied according to developmental stage and season (292 - 434μm). After a small decrease during cleavage, the egg diameter increased during embryonic development. The diameter just before hatching increased from September to April, but decreased from April to July. The minimum mean diameter during development was slightly smaller during the warmer season. The proportional increase in egg diameter (18 - 28%) was not very high at higher temperatures. During the warmer season smaller eggs were oviposited, and these increased proportionately less in size.