Chlorophyll content and biomass of plants are usually increased by the addition of certain amount of ammon-i um to the nutrient solution.In order to study the mechanisms of enhanced effects by ammonium on plant growth and chlorophyll content,Chinese cabbage was hydroponically grown in nutrient solutions with six different ratios of NO₃^-:NH₄⁺,i.e.,5.0:0.0,5.0:2.5,5.0:5.0,5.0:7.5,5.0:10.0 and 0.0:5.0.The results obtained were as follows.In the presence of 5 mmol L^-1 nitrate,addition of ammonium(2.5 mmol L^-1) to the solution promoted the biomass and leaf area of Chinese cabbage by 39.6% and 1613% compared with no ammonium addition and the leaf area was significantly correlated with biomass(r=0.941,p< 0.01).There was a significant relationships between ammonium concentration added and SPAD readings,active iron content and chloroplast protein content in the plants with the r values of 0.914,0.954 and 0.964,respectively.The mechanism of the properly enhanced ammonium on biomass was the facilitated expansion of leaf blade,and thus the increased total photosynthet ic area.The facilitated expansion of leaf blade can be attributed to leaf cell division resulting from the enhanced ammonium effect,which should be studied in future.A further study showed that the reason for enhanced ammonium to increase the chlorophyll content was related to the increased reusage of total iron in Chinese cabbage plant,thus increasing the active iron content in leaf blade and chloroplast protein content.