Autumn-morph adults of the Japanese common grass yellow, Eurema mandarina, overwinter with reproductive diapause, while the summer-morph adults do not. Because of different critical photoperiod between the sexes, the summer-morph males fly sympatrically with the autumn-morph adults in late autumn during which autumn-morph males show little mating activity. Consequently, mating between summer-morph males and autumn-morph females frequently occurs, and females stored sperm inseminated from summer-morph males throughout the winter. After overwintering, mated autumn-morph females commonly re-mate with autumn-morph males. Two hypotheses, a nuptial gift hypothesis and a risk-hedge hypothesis, explaining the adaptive significance of pre-overwinter copulation by autumn-morph females have been proposed. Then, timing of re-mating and the onset of oviposition after overwintering in relation to the population dynamics were examined by dissecting the autumn-morph females and by the mark-and-recapture method in the field. There were no mature eggs in the ovaries of the autumn-morph females in the autumn, though most of them already have a spermatophore derived from the summer-morph males. About 25 mature eggs were found in late March, indicating that egg maturation was occurred immediately after the overwintering. The decrease in the number of eggs contained in autumn-morph females indicated that they started oviposition in mid-April. The shape of the spermatophores in the bursa copulatrix suggested that most autumn-morph females re-mated with autumn-morph males before mid-April. Because autumn-morph females have sufficient chance to mate with autumn-morph males before the onset of oviposition after overwintering, the nuptial gift hypothesis is more plausible than the risk-hedge hypothesis. In addition, because the mortality of autumn-morph adults during overwintering was about 50 %, nutrition for overwinter must be important. High survival rate of autumn-morph females rather than autumn-morph males after overwintering suggested that the ejaculates transferred from males might be a nuptial gift as well as sperm supplying.