PAR (photosynthetic active radiation) is an important factor affecting crop growth, yield and nutrition status. The decrease in light density caused by shading often reduces dry-matter mass and nutrition status. But shading at the summer midday may not always cause decrease in nutrition status. The effects of three levels of shading (0, 40% and 75% shading) ratio on the nitrogen allocation in tomato plants were investigated at three growth stages ［early flowering (EF), peak flowering (PF) and late flowering (LF) stages］. Different effects caused by shading at different growth stages were found: Shading had no effect on the leaf N contents and N allocation index (NAI) at the EF and PF stages, while the leaf N contents and the NAI of leaf, stem and stubble tissues at the LF stage were higher than those at the EF and PF stages under 40%-shaded conditions, and higher yield was gained. The results suggested that the adverse influences caused by high irradiation and high temperatures could be overcome and a good nutrition status, higher yield also could be obtained if some moderate shading was applied at the LF stage during the summer midday time.