[Objectives]  To explore sexual size dimorphism in the Asian corn borer, Ostrinia furnacalis (Guenée), and investigate the relationship between larval time and pupal weights in different geographic populations of this species. [Methods]  The duration of the larval period, pupal weight, and adult weights of each sex were recorded in specimens obtained from four different geographic populations under a temperature of 26℃ and a photoperiod of L︰D=16︰8. Specimens were obtained from a tropical population from Sanya, Hainan Province, subtropical populations from Guangzhou, Guangdong Province and Yongxiu, Jiangxi Province, and a temperate population from Langfang, Hebei Province. [Results]  Females are consistently bigger than males in all four geographic populations. There was no significant difference in the duration of the larval period between females and males in the Sanya population, but female growth rate was significantly higher than that of males. There was no significant difference in the growth rate of females and males in specimens from the Guangzhou, Yongxiu and Langfang populations, but females took more time to complete larval development than males. Furthermore, pupal weight was negatively correlated with the duration of larval development. [Conclusion]  The Asian corn borer has significant sexual size dimorphism and geographic variation in both larval development and female and male growth rates.