Diffusion welding (DW) is a solid-state joining process, capable of joining a wide range of metal and ceramic combinations to produce both small and large components. In this study, low-carbon steel (AISI 1010) was joined to commercially pure copper by diffusion welding technique. Metallographic and mechanical examinations were carried out to determine the welding quality. Diffusion interfaces were investigated by scanning electron microscopy (SEM) and x-ray energy-dispersive spectroscopy (EDS) analysis in order to observe the effect of allotropic-transformation temperature on the amount of diffusing elements. It was found that Fe diffusion from AISI 1010 steel to copper at the transformation temperature of 845 degrees C was lower than the measurement at 770 degrees C. This low-diffusion amount at 845 degrees C was attributed to the consumption of the internal energy for the phase transformation. From the findings of this study, it can be concluded that the perfect diffusion welding can be done in the temperature just below the allotropic transformation. The shear strength in DW joints was also evaluated.