The printed circuit heat exchanger (PCHE) is a micro-channel heat exchanger with the benefits of high efficiency and can endure high temperature and pressure. Based on these benefits, PCHEs are widely used in marine engineering, nuclear engineering, and solar thermal system. When multiple cross-flow or serpentine passes are designed in heat transfer plate, longitudinal conduction due to temperature differences in the same heat plate and same flow channel can result in heat transfer reduce. This reduction was numerically studied in this chapter. The results show that longitudinal conduction does have a significant effect on heat transfer, and the existence of longitudinal conduction results in more than 40% heat load reduction. The influences on different Reynolds numbers also were studied. In order to reduce this reduction, we dug a rectangle groove on the heat plate and calculated the influence with various heights of the groove. We also calculated the influence with varying distances between adjacent flow channels, and 10 mm is a good choice.