Abstract: The quality penalty is usually taken as a measure to stimulate the supplier to improve quality. In practice the quality improvement usually involves the supplier and the purchaser. In order to find the effect of aftersale quality loss on the decisionmaking on quality improvement, a game model of decisionmaking on quality improvement was proposed. It is found that the purchaser can not necessarily stimulate the investment of the supplier into the quality improvement by increasing the proportion of the aftersale quality loss to be borne by the supplier, and that the purchaser shall not make the supplier solely liable for the quality improvement. In choosing a supplier, the purchaser shall not only focus on the commitment of the supplier to assume certain proportion of the aftersale loss, but pay due attention to the preliminary quality status and the decisionmaking on quality improvement of the supplier.