Clover seedlings were grown at different nitrogen concentrations (5, 10, 15, 20, 25 mM NO3 -, i.e. N5 to N25) and two irradiances, I (200 and 400 µmol m-2 s-1 of photon flux density, i.e. I 200 and I 400). Net photosynthetic rate (PN), photosynthetic nitrogen use efficiency (PNUE), leaf chlorophyll (Chl) content, maximum photochemical efficiency (Fv/Fm), and actual photochemical efficiency of photosystem 2 (PS2) (ΦPS2) increased from N5 to N15 and decreased with N15 to N25. P N, PNUE, and ΦPS2 were higher at I 400 than at I 200, but Fv/Fm and leaf Chl contents at I 400 were lower than at I 200. The effects of the N and I on specific leaf area (SLA) and N contents per unit dry mass (Nm) were similar, the SLA and Nm increased from N5 to N25 and they were higher at I 200 than at I 400. The nitrogen contents per unit area (Na) increased from N5 to N20, but decreased from N20 to N25. The Na was higher at I 200 than at I 400 when Trifolium repens grew at N5 and N10, but it was higher at I 400 than at I 200 at N15 to N25. and H. An, Z. P. Shangguan.