The effects of pulse modulation on particle growth by coagulation between particles in a pulsed SiH4 plasma reactor were analyzed by using a discrete-sectional method. At the start of the plasma discharge, there is high concentration of small-sized particles, and, later, the large-sized particles appear and grow by coagulation between smallsized particles. During plasma-off, the monomer generation stops and the particle concentration decreases with time by the effects of particle coagulation and fluid flow. As the pulse frequency decreases or as the duty ratio increases, the large-sized particles grow faster because more monomer particles are generated during longer plasma-on time. These results show that the pulse modulation, the changes of pulse frequency and duty ratio, can play a key role in suppressing the particle growth in the pulsed plasma process efficiently. This study proves that the pulsed plasma process can be applied to reduce the particle contamination in the plasma process for preparation of high quality thin films.