Egg maturation dynamics is closely associated with oviposition decision behaviors in parasitoids. So, it is necessary to understand egg maturation dynamics and factors that may affect it with the goal of revealing characteristics of reproduction ecology. The present study, using the gregarious parasitoid, Pteromalus puparum and host Pieris rapae pupa as the model system, was undertaken first to describe daily changes in external morphology of postembryonic stages, and then to observe daily variations in number of mature eggs in adult parasitoids emerged from host pupae that previously received the treatment of low temperature. The postembryonic development took about 2 weeks, where egg stage lasted for 1 day and larva 7 days. Neonate larva was clear, and grew rapidly with body color from green to yellow during next 3-6 days. Prepupal stage appeared 8 days later in light color, and the eye color changed from pale yellow to dark red 9-12 days later. Pupa was dark in color with metallic luster on day 13-14. Egg yolk appeared in ovarioles on the same day of adult emergence, and increased during a next few days until oösorption. Mature eggs did not obviously increase, or even decreased slightly during first 4-5 days after adult emergence. The host pupa, following development of the progeny parasitoid from egg to larva and pupa, varied in color from green to light brown and then to yellowish brown. Ovarioles were thinner and total matured eggs fewer in parasitoids receiving the low temperature (5℃) treatment than those from the control (25℃). Egg maturation was not only influenced by the low temperature, but also by female body size and age. The low temperature treatment markedly reduced egg maturation progress, resulting in fewer daily mature eggs than did the optimal temperature treatment. The peak time for mature eggs appeared on the 4^th day after adult emergence under the low temperature treatment, but as late as the 7^th day under the optimal temperature treatment. Number of mature eggs increased with female body size under both optimal and low temperature treatments. This study suggests that egg maturation dynamics can be affected not only by low temperature on parasitized host pupae but also by female body size and age.