We numerically calculated the effects of pulse modulation (plasma-on and -off times) on the concentration changes of the chemical species (SiH4, SiHx, SiHx+ and polymerized negative ions) and also the growth rate of a-Si : H thin films in the pulsed SiH4 plasmas. During the plasma-on, SiHx is generated quickly by a fast dissociative reaction of SiH4, but, during plasma-off, SiHx disappears rapidly by a reaction with hydrogen and also by the deposition onto the reactor wall. During the plasma-on, the negative ions are polymerized by the reactions with SiH4, but, during the plasma-off, they disappear by neutralization reactions with positive ions. As the plasma-on time increases or as the plasma-off time decreases, the time-averaged concentrations of SiHx and negative ions and also the time-averaged film growth rate increase. This study shows quantitatively that polymerized negative ions, which are not considered to be preferred precursors for the high-quality thin films, can be efficiently reduced by the pulsed plasma process.