The primary requirements of a secure Wireless Sensor Network architecture are confidentiality, integrity and authentication of users and other participating entities. User Authentication for wireless sensor networks is a fundamental and important issue in designing dependable and secure systems. In this thesis, we have outlined the security model, functional requirements, assumptions and network setup for an authentication scheme in the first phase. Keeping in mind the security requirements as well as the flaws of past authentication schemes, we propose a robust user authentication method that inherits user anonymity, mutual authentication and password changing functionality of previous password-based schemes and improves security by resisting gateway bypass and replay attack, and many logged in user with the same ID threat. Our scheme is a variant of strong password based schemes that does not require strict network synchronization.In the second phase of the thesis, we have analysed our authentication scheme from the perspective of security issues and functional requirements. The proposed scheme is modelled in SystemC. It is evaluated in different attack scenarios. The authentication latency, memory and functional requirements, and computational overhead are the metrics used to evaluate the scheme. The effect of multiple users on authentication latency in our scheme is also studied.Some of the past representative schemes have also been modelled and evaluated in the same environment. A detailed comparison of over-head cost, authentication latency and security features are provided in this thesis. It is verified and confirmed by modeling that our scheme provides enhanced security without adding extra computation at the sensor node.