An efficient online/offline signcryption scheme based on elliptic curve for MANET is proposed in this paper. The proposed scheme takes lower computation and communication cost, since most of computations are carried out offline and online part is very efficient. It not only provides the security properties of confidentiality, authentication, integrity, forward secrecy of message confidentiality and secure against man-in-the-middle attacks, but also solves the key distribution problem using a node own private key and then generates a session key without the certificate authority(CA). Our scheme is especially suitable for MANET because the online computation part is very efficient and does not need a pre-existing certification authority. Empirical studies are conducted using GloMoSim to evaluate the effectiveness of our scheme. The simulation results show that the scheme is efficient in terms of cost and overhead.