Abstract: 【Objective】The forest primary productivity is determined by canopy photosynthesis. There is a huge spatial heterogeneity of canopy photosynthesis.【Method】To better understand the spatial difference in photosynthesis at the canopy level, a study was conducted on the Larix principis-rupprechtii Mayr plantations with different ages(20-year-old and 40-year-old)in the Taiyue Mountain of Shanxi Province to analyze the photosynthetic characteristics by Li-6400. 【Result】The net photosynthetic rate(P_n)were significantly differences among the crown of upper layer, middle layer and under layer in the 20-year-old plantation, while there were no significant differences in P_n among the various orientations. There was the same true in the 40-year-old plantation. For 20-year-old larch, carboxylation efficiency(V_c), Intercellular CO₂ concentration(C_i), transpiration rate(T_r)were major factors influencing P_n, while for 40-year-old larch, V_c, stomatal conductance(G_s), vapor pressure deficit(p_VPD)were the main factors affecting P_n. There were no significant differences in the chlorophyll a, chlorophyll b and total chlorophyll among the various orientations and level in the 20 and 40-year-old larch. 【Conclusion】This study suggests that the spatial heterogeneity of the photosynthesis in the canopy should be considered when the forest productivity in the canopy level is simulated and estimated. It is important for the deduction from a single tree to stand and simulation.