In this study, the effect of the pattern on a heat exchanger on heat transfer was examined by using numerical modeling method. Two heat exchangers in the form of rectangular prisms were modeled and simulated by using the Ansys Fluent R17.2 program. All the surfaces of the first heat exchanger were flat and only the upper and lower surfaces of the second one were designed diagonally pattern. The effect of the diagonal pattern on the outlet temperature of the fluid, the amount of heat transfer, the flow rate, the flow field and the pressure variation were investigated. For both models, numerical solutions were made on different mass flow rate. As a result, when the outlet temperatures of the fluids were compared, the outlet temperature of the patterned heat exchanger was lower for all the mass flow rates examined. When examining the heat transfer in both models; it was seen that the heat transfer in the patterned heat exchanger was higher than in the flat one. When fluid velocities in heat exchangers were compared; it was seen that the flow velocities in the patterned heat exchanger increased in the patterned regions due to the periodic and regional cross-sectional narrowing caused by the patterns on the inner walls. In addition, when the effect of the patterns on the pressure variation was examined; the patterned heat exchanger was determined to have more pressure loss than the flat one due to diagonal patterns.