This paper introduces the payment process of SET(secure electronic transactions)protocol,and extends the logical framework of NDL to fit the purpose of logical verification of SET.This means to add some new axioms about SET protocols and redefine some given inference rules.Based on these,the logical verification for key fragments of SET protocol is given to show the importance of NDL and its extensions in E-commerce.Topics for further research on the “Rule of Accumulation” are also proposed.