Security schemes of pairwise key establishment plays a fundamental role in research on security issue in wireless sensor networks. However, establishing pairwise keys in wireless sensor networks is not a trivial task, particularly due to the resource constraints on sensors. In this paper, we prose an efficient key distribution scheme, in which each sensor node randomly selects two key pools from several different key pools and chooses keys from these key pools. The analysis shows that this proposed scheme can substantially improve the security of existing key predistribution scheme.