A block color image encryption algorithm based on chaotic system, elementary cellular automata (ECA), and deoxyribonucleic acid (DNA) sequence operations is proposed. The architecture is comprised of forward-diffusion, permutation, and backward-diffusion. First, the red, green, and blue components of a color image are converted into three DNA matrices by DNA encoding rule. Exclusive OR (XOR) operation is applied between the DNA coding chaotic sequence generated by the improved Henon map and three DNA matrices. Second, the large DNA matrix, composed of three DNA matrices, is divided into several small blocks with unequal sizes. ECA is employed to scramble the positions of these blocks. Finally, DNA computations are introduced to diffuse three permutated DNA matrices. In addition, initial values and system parameters of the chaotic system are calculated from the secure hash algorithm (SHA-256) hash value of the plain image and the given parameters. Both DNA encoding and ECA order rules rely on plaintext information. Therefore, the proposed algorithm is highly sensitive to plain image. Simulation results and security analyses demonstrate that our algorithm achieves excellent security performance and acceptable running time.