Abstract: The experiment was carried out in the pine plantation of Longshan forest farm in Zhejiang Province from March of 1983 to April of 1985. The damage caused by the pine caterpillar was simulated by artificial defoliation of the pine tree of different age classes in two kinds of soil conditions and under four seasonal periods. The orthogonal regression design was adopted in the experiment and data was analized by stepwise regression method. The results shown that in the first year after defoliation, the effect of defoliation on wood volume is less than the age classes of tree, i. e. the growth rate is regulated by tree itself. The relation between wood volume losses and needle losses is not a simple linear relation but rather a curved regression. Two years after defoliation, the effect of defoliation is not evident.