The hydrogenation mechanism has been studied in the hydrogen plasma annealing of a polycrystalline silicon (polysilicon). Hydrogen depth profile measurements reveal that hydrogen atoms are introduced into the polysilicon by diffusion in which the diffusion constant and the surface density of hydrogen atoms increase as the rf power rises. The density of dangling bonds measured by electron spin resonance (ESR) shows that hydrogen atoms passivate dangling bonds more effectively with a higher rf power. By using the polysilicon hydrogenated at various rf powers, thin‐film transistors (TFT’s) have been fabricated to investigate the effect of hydrogenation on the TFT field‐effect mobility. The results show that hydrogen plasma annealing with a rf power of 50 W increases the field‐effect mobility. However, a decrease of the field‐effect mobility, which is considered to be caused by plasma induced surface damage, is also observed when the rf power is higher than 50 W.