Electron spin resonance (ESR) is an exceptionally sensitive tool to investigate unpaired electrons such as charge carriers in organic semiconductors. Here we review the ESR analyses of charge carrier dynamics in high-mobility organic transistors on the basis of motional narrowing effect: the Brownian motion of carriers makes the ESR spectrum narrower. Particularly, the evaluation of the charge transfer rate between trap sites and/or crystalline domains enables us to understand the charge transport mechanism from a microscopic perspective. We also compare the temperature dependence of the charge transfer rate and field-effect mobility in order to discuss what limits the performance of organic transistors.