Abstract: The effects of different nitrogen forms (NH+4-N and NO-3-N) and ratios on the photosynthetic characteristics of the Pinus koraiensis seedlings were studied by pot experiment and the physiological mechanism was discussed. The results showed that the nitrogen treatments decreased the plasmalemma permeability and increased the net photosynthetic rate,the stomatal conductance, the RuBPcase activity, the sugar contents and the electron transport activities of PS Ⅰ and PS Ⅱ in comparison with the control. The increase of the physiological indexes mentioned above was the highest in the mixed treatment of NH+4-N and NO;-N, then was in the NO3--N treatment and the lowest in the NH4+-N treatment. The most benefit revealed as n(NH+4+-N)/n(NO;-N) ratio was 1/3～2/2. It is suggested that the rational ratios of different nitrogen forms be considered when the seedlings are to be fertilized.