Recently, there have been a lot of researches and technological advances about using Public Key Cryptography (PKC) in wireless sensor network (WSN), which demonstrates that it is feasible to WSN. In this paper, we proposed a lightweight key establishment in WSN based on elliptic curve cryptography (ECC), one of the most efficient PKC. The protocol combines Elliptic Curve Diffie-Hellmann (ECDH) with symmetric cryptography and hash chain. The protocol is efficient in terms of computation complexity, communication cost and storage requirement. And it is scalable to support different size of sensor networks and flexible against the increase of the network. Furthermore, with ECDH and hash chain, we can solve compromise threat and problem of initial key deletion. Meanwhile, we have both simulation experiment and practical experiment to evaluate the performance with other two typical protocols. It turns out that our protocol is more efficient than other public key schemes.