When mung bean seedlings were grown under natural condition for 12 days, leaves were cut off and floated on water. These detached leaves were treated with dark, white light and blue light respectively. The light intensity was 10 ± 2 μmol· m-2s-1. Some physiological parameters were determined during 8 days of treatment. As compared with the treatment in the dark, the treatments with blue and white lights were more effective in retarding the losses of chlorophyll and protein. However, there were no sig nif-icant difference in contents of these two substances under white and blue lights. In addition, blue light led up to the obvious opening of stomata and the increase of transpiration rate. After blue light illumination, SOD activity was maintained at a higher level and the increase of relative permeability of plasmalemma was retarded. The results indicate that blue light is able to retard the senescence in detached leaves of mung bean seedling.