Abstract: Application of chemical fertilizer, especially over-fertilization, conduced to the accumulation of nitrate nitrogen easily in the leaves and petioles of spinach . Literatures reported that K and Mo application could improve NRase activity of leaf which resulted in the reduction of nitrate content. However, the interactive effects of Mo and K on the accumulation of nitrate nitrogen in vegetables, and how to balance application of Mo and K was little known. Whereas, pot culture experiment was conducted to study the effects of fertilization (N, K and Mo combined application) on contents of nitrate nitrogen in the leaves and petioles at different growth stage of spinach. The objective of this study is exploring the optimal condition under which nitrate content decreased with Mo application. The results showed that the nitrate contents differed with positions. Nitrate content in petioles is the highest, followed with old leaves and young leaves. Compared with growing prophase of spinach, the contents of nitrate nitrogen in different position trended to decrease at harvest. Without the nitrogen application, the nitrate contents in spinach leaves were dramatically decreased by Mo application. The decreasing amplitudes were 19.3% at growth prophase and 21.4% at harvest, respectively. With the nitrogen application, the nitrate nitrogen contents in spinach leaves were only dramatically decreased by Mo application at growth prophase,the (decreasing) amplitude was 21.2%. However, Mo or K application could not significantly decrease the contents of (nitrate) (nitrogen) in spinach petioles. Single Mo application is more propitious to decrease the nitrate nitrogen contents in spinach leaves than Mo and K combined application. The interaction of Mo and K and how to balance fertilization maybe is key point which influence the effects of Mo application.