Population dynamics of the Oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), were monitored year-round using methyl eugenol-baited traps in 2003, 2004, 2005 and 2006 in Baoshanba, Yunnan Province, China. Environmental factors including air temperature, rainfall and host-plant species were analyzed with respect to the population dynamics. This species occurred only during April–November, with one yearly peak in August. The population fluctuation patterns with respect to season were identical in all study years. Correlation analysis and stepwise regression analysis indicated that air temperature, rainfall, sunlight hours and relative humidity were the major climatic factors that correlated with changes in the size of the fly population, and that monthly mean temperature, monthly sunlight hours and monthly relative humidity were most important. The seasonal increase in population size coincided with the fruiting period of the fly's host plants, but host fruit availability influenced the population size only when temperatures were sufficiently high. Cold temperatures may explain why there was no trap capture in the winter months. We believe that air temperature is the key factor explaining the seasonal occurrence of the fly population at Baoshanba.