Certificateless public key cryptography (CL-PKC) is a useful method in order to solve the problems of traditional public key infrastructure (i.e., large amount of computation, storage and communication costs for managing certificates) and ID-based public key cryptography (i.e., key escrow problem), simultaneously. A signcryption scheme is an important primitive in cryptographic protocols which provides the goals of signing and encrypting, simultaneously. In 2010, Liu et al. presented the first certificateless signcryption (CLSC) scheme in the standard model, but their scheme is vulnerable against different attacks presented in the literature, till now. In this paper, we improve their scheme and propose a new CLSC scheme, which is semantically secure against adaptive chosen ciphertext attack under the (Ѕ2, 5)-BDHE-Set assumption and existentially unforgeable against adaptive chosen message attack under the 3-CDHE assumption in the standard model. Our scheme is more efficient than all other secure CLSC schemes in the standard model proposed up to now.