Slow slip behaviors are suggested to have a close correlation with the presence of excess pore fluid pressure. In this study, we conducted deformation experiments with and without excess pore pressure on intact porous sandstone samples to investigate effect of pore fluid pressure on rupture growth and slip instability. Experimental conditions are such that the samples failed either in brittle faulting or transitional regimes. The experimental results indicate that in the brittle faulting regime prone to seismic deformation, excess pore pressure causes reduction in brittle strength but no detectable difference in slip behavior compared to the cases without excess pore pressure. In the transitional regime, which is prone to stable deformation, excess pore pressure induces slip instability on otherwise stable sliding faults, but there are measurable differences compared to the unstable slip in the brittle faulting regime due to the interaction between unstable crack growth and dilatancy hardening.