In order to study the effects of pH and the ratio of nitrate to ammonium in nutrient solution, on the growth characteristics and on the yield of spinach (Spinacia oleracea L.), a factorial experiment was conducted consisting of two factors including: nutrient solution pH in three levels (4.5, 6.5 and 8) and nitrate to ammonium ratio of nutrient solution in five levels (100:0, 75:25, 50:50, 25:75 and 0:100). The experiment was carried out in a completely randomized design with four replications in perlite culture in greenhouse conditions. The results indicated that by decreasing the ratio of nitrate to ammonium in nutrient solution, leaf chlorophyll index was increased. By decreasing the nitrate to ammonium ratio from 100:0 to 75:25, net photosynthetic rate, leaf number, leaf area, fresh and dry weights of shoots were increased. With further decrease in nitrate to ammonium ratio, the above factors were decreased. By increasing the nutrient solution pH from 4.5 to 8, leaf chlorophyll index, net photosynthetic rate, leaf area and fresh as well as dry weights of shoots were decreased. The interactive effects of pH and the ratio of nitrate to ammonium in the nutrient solution on net photosynthetic rate, leaf number, leaf area and fresh and dry weight of shoots were significant, so that a pH of 4.5 with nitrate to ammonium ratios of 100:0 and 75:25, pH 6.5 with nitrate to ammonium ratio of 75:25, and pH 8 with nitrate to ammonium ratios of 75:25 and 50:50 yielded a the maximum dry weight of shoots.