Abstract: The geographic variation of photoperiodic response was studied in the pine caterpillar (Dendrolimus spp., in short: pc) at latitudes from 29.4 to 39.8N in China. The result demonstrated that the critical photoperiod for diapause induction observed at 27~28℃ was 14 h 30 min for Beijing strain (39.8N), 14 h 2 min for Jincheng (35.5N, Shanxi Province) and 13 h 24 min for Yongchuan (29.4N, Sichuan Province). The critical photoperiod increases about 30 min for each increase in latitude of 5N.Sensitivity of pc to photoperiod during the larval stage varied with age and latitude. The overwinter larvae originated from North China displayed a stronger diapause and can usually survive a longer period under 28℃ but those originated from the South showed a weaker diapause and can only survive a shorter period at the same temperature.It was revealed that photoperiodic diapause is the key factor to control the differentiation of generation in pc. Under shortdays condition, the larvae entered diapause in the third and fourth instar and were unable to continue development without terminating diapause even under a high temperature condition. When the natural photoperiod exposed to the larvae equal to its critical photoperiod the day reaches the critical point (date) of differentiation. The critical date plays a decisive role to determine the ratio of differentiation of pc. Moreover, temperature and nutrition can modify the ratio by affecting the critical date. The larvae hatched before the critical date can produce one generation more than those hatched after the critical date.In this paper, the relation between the enviromental factors and the ratio of the third generation of pc was discussed in Jinsishan Forest Farm (in Anhui Province).