Wireless Sensor Networks are susceptible to various types of security attacks in many ways both inside and outside the network. There have been many approaches researched to tackle the issue. In this paper, the authors propose MultiSEC: an efficient Secret Key Protocol which combines the benefits of both key management and malicious node detection to improve the security of wireless sensor network. A two-level secret key policy is adopted to improve the security in terms of encryption and authentication. Behavior based detection algorithm is introduced to determine the presence of malicious nodes. Simulation results show that the proposed algorithm achieves more than 90% of detection ratio with minimum computation, delay, packet loss and communication overhead.