Abstract: Accurate estimate of population density in integrated control of the pine caterpillar (Dendrolimus punctatus Walker)is one of the problems that need for urgent solution. In traditional methods the larvae in a tree were counted directly with the naked eye. It is difficult and subject to great errors to use the traditional methods in case of high trees. The relationship between frass-drop as well as related factors and population density ef the pest were investigated in Long-Shan Forestry Farm of Anji County and in Sian Forestry Farm of Chang-xing County, Zhejiang Province.The frass-drops were collected at different stands under different weather conditions. After collecting the frass-drops, the larvae living in the branches above the collector were dislodged by both chemical knockdown and jarring the tranks. Both the frass-drops and the larvae were counted. Many factors regarding time and space were recorded, which probablly affect the relationship between the frass-drop amount and the population density. The data were analysed by simple regression and stepwise regression. A series of mathematical models are developed in this paper.