Abstract: A block image encryption algorithm with a 256 bit key was designed based on a logistic map and a spatiotemporal chaotic system.The algorithm encrypts a block of original image into a block of cipher image with the same length of 256 bit.An auxiliary key brought in and iterations designed in the algorithm are sensitive to the original image and secret keys.An alternating iterative chaotic system and an Arnold map disturb pixel values and shuffle the positions of pixels.The results of computer simulation and cryptanalysis show that the algorithm is characteristic of excellent sensitivity to the original image and keys,large secret key space and high expansibility.The algorithm also has excellent effective encryption and strong anti-attacking performance.It is indicated that the algorithm is suitable for security communication.