Wireless Sensor Network consisting of a large number of sensor nodes that connected through wireless media has emerged as a ground breaking technology that offers unprecedented ability to monitor the physical word accurately. The privacy preservation is an important issue in wireless sensor network. Developing effective security solutions for wireless sensor networks are not easy due to limited resources. In this paper we propose new techniques for the purpose of security in wireless sensor network called as SDEP sensor data encryption protocol. In the scheme we use the RC 6 method for the purpose of encryption and decryption. RC 6 provide best confusion and diffusion properties with the less computational overhead. In order to confirm effectiveness of SDEP, a comparative performance evaluation with AES and RC 5 algorithms are presented in terms of memory requirement and execution time criteria. Our proposed scheme provides better performance than AES and RC 5 in the term of execution time and total memory requirement. We also provide simulation results for proposed method in the term of overhead and energy according to this result SDEP is strong block cipher for wireless sensor networks.