Growth and yield of plants are increased when plants are provided with mixtures of nitrate and ammonium compared with either form alone. Therefore, the objective of this experiment was determine the optimum of nitrate and ammonium ratio caused an increased in growth and yield of green pak choy (Brassica chinensis L.) The experiment was designed in Randomized Completely Design with five treatments of nitrate ammonium ratios and arranged in four replication. The treatments of nitrate ammonium ratio were : 100/0; 75/25; 50/50; 25/75 and 0/100. The results showed that nitrogen fertilizer applied in mixture nitrate and ammonium gave different effects in leaf nitrate reductase activity, leaf nitrogen content, growth and yield of green pak choy. There was significant correlation between the leaf nitrate reductase activity and leaf nitrogen content. There was also significant correlation between the leaf nitrate reductase activity with growth and yield of green pak choy. Nitrate ammonium ratio at 75/25 and 50/50 have better affect on the leaf nitrate reductase activity, leaf nitrogen content, growth and yield of green pak choy