The aims of this research were (1)to investigate the rainfall interception loss by trees at forest stands and (2) to develop a simulation model for estimating the amount of rainfall interception loss. A simulation model would be used for predicting the short-term effects of the change in the forest stand structures on water resources. For experimental purposes, two types of sample plots were established in Kwanak Arboretum of Seoul National University. One is located in Quercus mongolica stand and the other in Pinus rigida × Pinus taeda stand. In both plots, rainfall, throughfall and stemflow were measured for two rainy seasons, July to September of 1986 and July to October of 1981. The simulation model was composed of the stand structure parameters and Penman-Monteith equation for estimating the potential evaporation. The estimated data were compared with the measured ones. The results obtained from this research showed tendencies as follows : (1) The amount of rainfall interception loss at Pinus rigida × Pinus taeda stand was 24.7% more than that at Quercus mongolica stand. (2) The amounts of throughfall, stemflow and interception loss linearly increased as the rainfall amount augmented. (3) Because of the loss by evaporation, the longer duration of rainfall increased the amount of interception loss. On the other hand, interception loss was reduced exponentially as the intensity of rainfall increased. These measured data sets collected in 1986 were used in Constructing a simulation model to estimate the interception loss in the corresponding stand types as a function of the amount of rainfall, throughfall and stemflow. In building the simulation model, the potential evaporation was estimated using Penman -Monteith equation. A linear equation to estimate drip through canopy was derived as a function of the canopy storage. The results if test runs of the model indicated -5.1% and -30.5% simulation error ratios for Quercus mongolica stand data and Pinus rigida × Pinus taeda stand data, respectively. The simulation model was improved by applying the values of stand structure parameters, which were adjusted in a manner to minimize the simulation error ratio by a curve fitting method. In determining the adjusted values of the parameters, it was found that the stemflow coefficient and drip coefficient were very sensitive to the simulation error ratio. The improved model provided good estimations of interception loss to reduce the simulation error ratio to 0 and -1.5% for Quercus mongolica stand and Pinus rigida × Plnus taeda stand, respectively. The effects of the changes in the value of the stand structure parameters on the interception loss were investigated using various combination data sets of free throughfall coefficients, ranging from 0.5 to 0.0, and canopy storage capacity, ranging from 0.5㎜ to 2.1㎜. The results indicated that a stand with 0.0 tree throughfall coefficient and 2.1㎜ canopy storage capacity intercepted 30.8% more than a stand with 0.5 free throughfall coefficient and 0.5mm canopy storage capacity. The capability of the simulation model to estimate interception loss of forest stand would be useful in predicting the short-term effects of the change in forest stand structures on water resources.