The Secure Electronic Transaction (SET) protocol has been jointly developed by Visa and MasterCard toward achieving secure online-transactions. This paper presents formal verification of the Purchase Request phase of SET, by using ENDL (extension of non-monotonic logic). The analysis unveils some potential flaws. To overcome these vulnerabilities, some feasible countermeasures are proposed accordingly during the validation. Also, the modelling of Purchase Request is described to implement the mechanically model checking instead of manual verification.