An analysis of the crossflow plate-fin, crossflow tube-fin, counterflow plate-fm and parallel flow plate-fin compact heat exchangers accounting for the effect of ’longitudinal heat conduction’ through the exchanger wall is carried out using a finite element method. The exchanger effectiveness and its deterioration due to the longitudinal heat conduction effect have been calculated for various designs and operating conditions of the exchanger. The results indicate that the thermal performance deterioration of crossflow plate-fin, crossflow tube-fin and counterflow plate fin heat exchangers due to longitudinal heat conduction may become significant especially when the fluid capacity rate ratio is equal to one and when the longitudinal heat conduction parameter is large.