Interstitial fluid pressure is hypothesized to exert important controls on fault slip behaviors in several tectonic settings. In the classic model, the effect of pore pressure on fault frictional strength is quantified by the effective pressure law. However, there are relatively few constraints on whether and how pore pressure affects fault slip behavior. A recent study shows that brittle failure of intact antigorite-rich serpentinite changes from dynamic faulting at low pore pressure to stable sliding at high pore pressure for the same effective pressure (French and Zhu, 2017). Here, we investigated the influence of pore pressure on the frictional strength and slip behavior of serpentine gouge. We sheared layers of antigorite gouge with grain size from 63 to 120 μm between saw-cut surfaces of porous sandstone or steel cylinders in conventional tri-axial compression experiments. Experiments were conducted at pore pressures of 5 to 60 MPa, constant effective pressure of 70 MPa, and room temperature. Velocity-steps were performed to determine rate-and-state friction parameters. For comparison, additional experiments were conducted on olivine, quartz and chrysotile gouges. Antigorite gouge has lower friction coefficient ( 0.56 to 0.61) than olivine and quartz gouges ( 0.64 to 0.70). Chrysotile gouge shows the lowest friction coefficient ( 0.32 to 0.38). Whereas antigorite gouge exhibits both strain weakening and velocity weakening, chrysotile gouge is strain weakening but velocity strengthening. In contrast, olivine and quartz gouges are both strain hardening and velocity strengthening. Thus, serpentinization reduces fault frictional strength and favors velocity weakening when antigorite is present. For all gouges, higher pore pressures lead to more strengthening behaviors with the most significant effect on antigorite gouge. On the basis of measured pore volume changes, we postulate that dilatancy hardening is responsible for the observed strengthening, consistent with results from intact rocks. Our results indicate that high fluid pressure may cause a transition from potentially unstable fault slip to stable or slow slip behavior. These data provide new experimental constraints to better understand the seismic activities at plate boundaries where serpentinization takes place pervasively.