Changes in the grain size of tin (Sn) crystals associated with a thermal cycling treatment were investigated for layers of Sn-2 mass%Bi electroplated on Fe-42Ni substrates in order to clarify the effects of post-plating reflow. Morphology of plated surface was observed by scanning electron microscopy (SEM) and scanning ion microscopy (SIM), and the Sn-grains were observed by electron beam back scattered diffraction (EBSD). Microstructures were also analyzed with a transmission electron microscope (TEM) in order to investigate the process of whisker formation. It was found that the Sn grains in Sn-Bi specimen are smaller than those in pure Sn specimen at the initial state of as-plating, after the reflow treatment and after the thermal cycling treatment of 1500 cycles. On the Sn-Bi specimen without a reflow treatment, short whiskers had grown after 100 cycles of thermal cycling treatment, but on those with the reflow treatment, relatively-long whiskers were formed after 500 cycles. The grain sizes increase temporarily after the reflow treatment and also after the very early stage of thermal cycling treatment, but gradually decreased with the thermal cycling treatment. The phenomenon is explained by the ‘‘deformation and recrystallization’’ effects of the thermal cycling treatment.