We conducted experiments to investigate the influence of pore fluid pressure on the frictional strength and slip behavior of gouge bearing faults. Saw-cut porous sandstone samples with a layer of gouge powders placed between the pre-cut surfaces were deformed in the conventional tri-axial loading configuration. A series of velocity-step tests was performed to measure the response of the friction coefficient to variations in sliding velocity. Pore volume changes were monitored during shearing of the gouge. Our results demonstrate that increasing pore pressure stabilizes the frictional slip of faults with all 4 gouge materials including antigorite, olivine, quartz and chrysotile. The stabilizing effect is the strongest in antigorite gouge, which exhibits a transition from velocity-weakening to velocity-strengthening behavior with increasing pore pressure. The stabilizing effect is positively correlated with pore volume increases, suggesting that dilatant hardening is responsible for the observed strengthening. These results provide new physical understanding of the observed spatial correlation between slow slip events and high pore pressure in many subduction zones.