Broadcast authentication is a security mechanism in Wireless Sensor Networks (WSNs), which enables a lot of entities to join in and disseminate the messages into the networks in an authenticated way. Hitherto, some symmetric key based broadcast authentication schemes such as μTESLA and its variants have been proposed to provide such services for wireless sensor networks. Unfortunately, these schemes suffer from serious Denial of Service (DoS) attack due to the time delay in message authentication. In this paper, we propose a certificateless signature based broadcast authentication scheme without bilinear pairing in wireless sensor networks. Our proposal is proved to be secure in the random oracle model under the hardness assumption of discrete logarithm problem. Compared with the previous protocols, the perform evaluations show that this scheme is efficient and appropriate to be applied in wireless sensor networks.