Abstract: To quantify the competition from above-and-below-ground plants,an experiment was conducted by use of seedlings of Idesia polycarpa. Path analysis was employed to study both the relationships between below-and-above-ground competitions, and their contribution to general competitions among individual seedlings at 3 different planting densities. The results showed that there were close relationships between below-ground biomass,above-ground biomass and total mass of Idesia polycarpa seedlings and the growing densities. With the decrease of the density,the seedling biomass increased while each part of the competition index fell. The above-ground competition index was greater than the underground at the same planting density. Due to the interactions between each part as the plants grew,the correlations among competition indexes were significant coefficients of path analysis,at 3 planting densities,showing that the direct influence intensity of above-ground competition on general competition wafted from 0.5536 to 0.6533,significantly greater than 0.4802~0.6228 of the underground competition. With the decrease of the density, the above-ground-and-below-ground competitions were intense, however, the above-ground competition had an important influence on the general competition.