Abstract: The paper focuses on the correlation property of the combiners with memory.Using the hidden Markov model，a formula is given to compute directly the conditional probability of the memory state.Under some other conditions，we pay special attention to the usage of the formula in detail.Based on the conclusions obtained above，a conditional correlation attack is given to the improved summation generator.The comparisons with other attacks show that our attack offers good trade-off between the complexity and the required key stream length.