Close observation with a transmission electron microscope (TEM) and a scanning electron microscope (SEM) was performed for the growth process of tin (Sn) whiskers on lead (Pb)-free Sn-plating. Whiskers were formed on a Sn layer plated on Cu/polyimide flexible substrate. The whisker was found to be of a single crystal and have a characteristic "Y"-shaped grain boundary structure at its root. The growth process of a curling whisker was successfully observed in a continuous way in SEM. TEM observation revealed that the curling whisker had a single crystallographic orientation irrespective with its external shape. these microstructures indicate that the rearrangement of dislocations plays an important role in the growth process of whiskers.