Balance of soil N,P and K is very important to the nutrient management and agricultural sustainability.It was monitored in a small-scale vegetable farming system in the peri-urban areas of Nanjing and Wuxi,China during July 2003-July 2004,and its affecting factors studied at the same time.Results indicated that in Nanjing organic manure was the main source of nutrient supply,while in Wuxi,chemical fertilizer and organic manure were both used,with the former being the dominant one.Balancing of soil nutrients revealed that soil N and P gained,more in Nanjing than in Wuxi in the case of the former,and more in Wuxi than in Nanjing in the case of the latter,while soil K was in positive balance in Nanjing and in weak positive balance or negative balance in Wuxi,where obvious soil K deficit began to appear.This kind of soil nutrient imbalance resulted from differences in type of fertilizers used,labor input and resultant farming management between the two systems.Therefore it is necessary to develop a rational fertilization technique in light of soil nutrient balance and it's affecting factors,so as to promote sustainable development of the agriculture in the periurban areas.