We present the design, implementation and simulation of a security protocol based on Extensible Authentication Protocol nick-named EAP-Sens for wireless sensor networks. We use the generalized pre-shared key authentication method for authentication and key establishment. Standard EAP model is used for authenticating sensor nodes within the radio range of the authenticator. For distant nodes, we use Relay Authenticators to tunnel EAP messages to the authenticator. We have defined EAP message encapsulations for IEEE 802.15.4 standard and a key hierarchy for EAP-Sens. To analyze EAP-Sens performance on larger configurations a simulation model has been developed. We have implemented an EAP-Sens prototype for Tmote sensors and showed that EAP based security protocol is feasible in wireless sensor networks.