Annual monitoring was conducted on the population dynamics of the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) through methyl eugenol-baited traps all year during 1997, 2000, 2003 and 2004 respectively. Temperature, rainfall and host-plant species were analyzed in relation to population fluctuation of the fly. During the study periods the fruit fly occurred all year. Its population remained low from November to January and increased steadily from February until reaching a peak in June. Afterwards, the population declined until October. The results of stepwise regression analysis indicated that monthly mean temperature, monthly mean maximum temperature, monthly mean minimum temperature, monthly extreme maximum temperature, monthly extreme minimum temperature, and monthly raining days were the major climatic factors influencing populations. Path and decision coefficient analyses indicated that the monthly mean temperature was the crucial factor influencing population fluctuation, the monthly mean minimum temperature was the crucial limiting factor indirectly influencing population increase, and monthly raining days the strongest influence on fly population dynamics. Generally, the monthly mean temperatures fell into the ranges of temperatures suitable for development and reproduction of the fly. But the monthly mean minimum temperatures from November to January appeared to be lower and were suggested to be responsible for the low populations in this period. Monthly rainfall and rainy days increased steadily from February through June, which explained population increase. During periods of continuous heavy raining from July through August, the fruit fly population decreased remarkably. Host plant species was another essential factor influencing the population fluctuations. Abundant fruit and melon species provided the fly with a continual supply of the food and breeding material during the study periods.