Application of embedded systems is faced with multiple threats against security. To solve this problem, this article proposes a new program memory encryption mechanism (PEM) to enhance the security of embedded processor. The new mechanism encrypts all the programs via a secure cache structure. It not only caches the instructions read from the off-chip memory, but also stores the pad values used to encrypt the plaintext. It effectively accelerates encryption and reduces the performance overhead. Besides the encryption, PEM also monitors the program modifications and reset behaviors to reduce the risk of vicious tamper. The experiment indicates that PEM has an average of 2.3% performance improvement and results in a 25.71 % power reduction in the write-back stage. The new scheme offers a good balance between performance and security. It is fully practicable for embedded processor.