Abstract: In this paper, we present a secret key stream generating algorithm based on discrete chaotic systems. Using a simple chaotic system and another chaotic system based on linear system with a piecewise linear state feedback, we put the two sequences through threshold function and XOR operation. The result we get is an unpredictable pseudo-random sequence. The analysis and simulation results indicate that the algorithm can effectively resist the predictive attack and statistic analysis. This algorithm can meet the request for secret key stream in cryptography. So, it can be used for generating secret key stream in stream cipher.