Pulse widths of P waves in granite, measured in the laboratory, were analyzed to investigate source durations of rupture processes for water-pressure induced and stress-induced microseismicity. Water was injected into a dry granite sample under constant axial stress of about 70% of fracture strength and a confining pressure of 40 MPa. After the effects of event size and hypocentral distance were removed from observed pulse widths, the ratio of the scaled source durations of water-pressure induced and stress-induced microseismicity was 0.52. The difference in the scaled source durations between water-pressure induced and stress-induced microseismicity suggests that water-pressure induced microseismicity involves a greater rupture velocity or a more equidimensional fault geometry than stress-induced microseismicity. These results suggest that pulse width analysis of P waveforms can be used to distinguish water-pressure induced events from those induced by regional stress and to characterize the faulting process.