The xylem sap was collected in two growing stages to investigate the effects of enhanced ammonium nutrition (25% replacement of NO3--N by NH4+-N) on the contents of NH4+, NO3-, Ca, Mg, K, P, soluble carbohydrate, amino acid, and organic acids during tomato maturation. The results showed that the contents of K, NH4+, amino acid, soluble carbohydrate and malate, the ratio of amino acid content to NO3- in xylem sap increased significantly under enhanced ammonium nutrition in young fruit stage, while NO3- content decreased significantly. No significant effect on the contents of Ca, Mg and P, which indicated that comparing to the single nitrate in solution, enhanced ammonium nutrition could not only improve the root activity in absorbing enough K to meet the requirement of fruit development, but also increase the ability of N metabolism.