What is an ideal authentication method for smartphone banking services? And what are the critical elements to be considered when designing it? To provide valuable insight for these questions, this study investigates various authentication requirements to be considered in smartphone banking service with the aspect of security, convenience and cost. By applying Analytic Network Process (ANP), this study first analyzes priorities among the requirements and then draws an ideal authentication method for smartphone banking service. Moreover, a sensitivity analysis has been conducted by varying the relative importance of several requirements. The results from the judgment of 72 experts revealed that, although Korean government has obliged the use of Public Key certificate, OPT and biometric alternatives may prove to be more appropriate for the smartphone banking service. These results will contribute to the provision of more secured and convenient smartphone banking services.