The value of the optimum leaf area index (LAIopt), at which the net photosynthesis of a population (Pp) or foliage (Pf) was maximized, was investigated for mulberry using the simulation program designated as MORUS-LICS, in which a model mulberry population is generated and its light-intercepting conditions and net photosynthesis are predicted through the method of numerical experiment. Under the averaged radiation and temperature conditions in Central Japan in July and September, simulations predicted that the LAIopt was never constant and depended on the planting density. In a normally planted population, Pp increased until the time when the longest shoot length (LSL) and LAI reached values of 210 cm and 8.5, respectively, and thus the LAIopt could not determined in the course of shoot elongation. However, it was predicted that Pf would increase slightly by pruning when the LSL exceeded 150 cm. In this case, the LAIopt after pruning was in the range of 4.9-5.8. The densely planted population, on the contrary, became over-luxuriant at 120 cm LSL (LAI = 6.1), and Pp stopped increasing. Moreover, Pf showed a considerable increme　nt when the population with an LSL of 120 or 150 cm was pruned. The LAIopt values after pruning were 3.8 or 5.1, respectively.