Abstract: The strength of the samples with different fault gouges was studied under different pressure,temperature and pore pressure.The fault gouges included crushed granite,serpentine,illite and a natural fault gouge (DLV),collected from borehole along the San Andreas fault.Based on the response of the gouges to the pressure,temperature and pore pressure,the fault gouges were classified into two groups:the gouges of clastic type and the gouges of clay type.For the gouge of clastic type,such as crushed granite,its strength was very sensitive to the pressure.When the confining pressure increased 1kb,the strength of the gouges got a raise of 1.47 kb.It hasn't seen any response of the gouge to the temperature.Its response to the pore pressure is consistent with the effective pressure law.The rest three are gouges of clay type.The general character of them are as follows.There was an outstanding effect of the temperature on the mechanical behavior of the clay type gouges.The change of pore pressure exerted influence on the trend of the response to the temperature.The effect of pressure for clay type gouges only can be seen when the temperature is lower than 200℃. The results of X-ray diffraction indicate that no mineral change has been found in crushed granite gouge up to 600℃ and the strength of it was controlled by the compaction.The mechanial behavior of clay type gouge reflect the change of clay mineral composition,structural state of gouge material and probably,the change of permeability.Pore pressure has an effect on the chemical process.So it is clear that need pay attention to the effect of the physico-chemical environment on the strength.