Abstract: In this paper the authors first present a related-key attack on chaotic stream ciphers. Combining the idea of linear cryptanalysis and divide-and-conquer attack on chaotic ciphers, the attack uses the output sequences created by multiple related keys, hence enhances the efficiency of divide-and-conquer attacks. The related-key attack breaks the confinements that it is difficult to reduce the computing complexity of divide-and-conquer attacks to a realizable range when the information amount leaked by the distribution of coincidence degree of a chaotic cipher is small or the size of key is large. As an example, the related-key attack on the ZLL chaotic cipher with 64 bits key on a Pentium 4/2.5GHz personal computer was realized and it took about 154s to recover key at a success rate 0.96.