The deterioration of the performance of a cross flow heat exchanger with condensation due to the longitudinal conduction was investigated. For a relatively small change of wall temperature along the flow directions which was shown to be common in many applications, the energy equations were solved numerically. An iterative method was used to solve the resulting equations of the finite difference form in Order to obtain the hot fluid, the cold fluid and the wall temperature distributions and the exchanger thermal effectiveness. A comparison was made between the effects of longitudinal conduction on the exchanger performance, without and with condensation. The results indicated that with the same values of NTU, the capacity ratio and the conduction parameter, λ, the performance deterioration of a cross flow heat exchanger with condensation is not as strong as the case of pure heat transfer.