Whisker is a single beard-shape crystal filament, and it would be a main cause of producing electrical short circuits. Re-crystallization of both plating grain and base metal grain caused by internal and external stresses affects whisker growth. In this paper, we analyzed effect of grain size and structure of plating on whisker growth through whisker acceleration tests of ICs used in the electronic products. Samples with different package types, base metal, and plating thickness were used in the tests, and both structure and size of plating, and whisker were examined using SEM. From the test results, internal and external, stresses produce more grain boundaries of plating when the grain size is smaller. And based on t-test, we found that there is a significant negative correlation between grain size and whisker growth. Therefore, smaller grain size of plating produces higher possibility of whisker occurrence, and whiskers grow in the grain boundaries of plating.