A well designed and implemented working capital management is expected to contribute positively to the creation of a firm's value. The purpose of this paper is to provide empirical evidence about the effects of working capital management on firm's performance. We first give literature review and develop four Hypotheses. The dependent variable, return on core asset is used as a measure of profitability, and the key variables used as measures of working capital management efficiency in the analysis are inventories days, accounts receivables days, accounts payable days and cash conversion cycle. We assume inventories days, accounts receivables days and cash conversion cycle are negatively related to corporate profitability, while accounts payable days are positively related to corporate profitability. Then the relationship between working capital management and corporate profitability is investigated for a sample of 519 Chinese Listed Manufacturing Companies, using panel data analysis for the period of 2004-2006. The regression results show that there is a statistical significance between profitability and working capital management efficiency measured by cash conversion cycle and each different component. High investment in receivables and inventories is associated with lower profitability, and short cash conversion cycle is associated with higher profitability. However, there is a negative relation between accounts payable and corporate profitability, which is not consistent with our hypothesis. A possible explanation for that is in the special environment of emerging economy in China, less profitable firms may wait longer to pay their bills. The results suggest that managers can increase corporate profitability by setting up proper trade credit policy and inventory policy to handle correctly the cash conversion cycle and keep accounts receivables, accounts payables and inventory to an optimum level. Although the conclusion is drawn from analyzing data of listed manufacturing companies, it is meaningful for listed companies in other industries and small enterprises as well.