Non-repudiation of a mobile payment transaction ensures that when a buyer (B) sends some messages to a seller (S), neither B nor S can deny having participated in this transaction. An evidence of a transaction is generated by wireless PKI (WPKI) mechanism such that B and S cannot repudiate sending and receiving the purchase order respectively. Broker generates a mobile agent for B which carries encrypted purchase order to S. A trusted third party (TTP) acts as a lightweight notary for evidence generations. One advantage of this agent-based non-repudiation protocol is to reduce inconvenience for mobile clients such as connection time and search for suitable merchant servers, etc.; it provides necessary security mechanisms for fair mobile payment transactions.