In this paper, the responses of larch seedling's photosynthetic characteristics to N and P deficiency were studied in greenhouse from April to September, 2002. On April 10, one-year-old larch seedlings, after their roots washed and sterilized, were transplanted into a plastic pot (diameter 30 cm, height 27 cm) containing sterilized and washed quartz sand, and supplied with complete nutrient solution (4 mmol NH4NO3 x L(-1), 1 mmol KH2PO4 x L(-1), 1 mmol KCl x L(-1), 1 mmol CaCl2 x L(-1), 0.6 mmol MgS4 x L(-1), 0.02 mmol FeCl3 x L(-1), 6 micromol MnC2 x L(-1), 0.016 mmol H3BO3 x L(-1), 0.3 micromol ZnCl2 x L(-1), 0.3 micromol CuCl2 x L(-1), and 0.3 micromol NaMoO4 x L(-1)). On May 20, the seedlings were treated with four different combinations of N and P supply, i.e., control (CK, 8 mmol N x L(-1) and 1 mmol P x L(-1)), low nitrogen supply (LN, 1 mmol N x L(-1) and 1 mmol P x L(-1)), low phosphorus supply (LP, 8 mmol N x L(-1) and 1/8 mmol P x L(-1)), and low nitrogen with low phosphorus supply (LN-LP, 1 mmol N x L(-1) and 1/8 mmol P x L(-1)). In treatment LN, the other nutrients were given as CK, while in treatment LP, the K level was adjusted by KCl, with the other nutrients kept as CK. On September 4, the net photosynthetic rate, chlorophyll fluorescence parameters, chlorophyll content, total soluble protein content, and N and P contents of the needle were determined. The results showed that in treatment LN, the N content, chlorophyll content, net photosynthetic rate (Pn), maximal PSII efficiency (Fv/Fm) and potential PSII activity (Fv/Fo) of the needle declined significantly by 37%, 31%, 58%, 22% and 57%, respectively, while in treatment LP, the N and P contents, chlorophyll content, total soluble protein content and Fv/Fm had no significant differences with CK, but the Pn and Fv/Fo was 13% lower than CK. The variation tendency and extent of Pn, chlorophyll fluorescence parameters, and other physiological indices in treatment LN-LP were similar to those in treatment LN.