Abstract: The mechanical properties of weak porous sandstone with uniaxial compressive strength of about 300kg/cm2 , porosity as high as 27% and bulk density 1.85 g/cm3 , were investigated experimentally by using Mogil’s true triaxial compressive apparatus. The properties of deformation and failure, in the whole process from brittleness to ductility, were observed by bringing confining pressure up to 500 kg/cm2 under room temperature . The intermediate principal stress was successfully brought up from the lowest limit. σ2=σ3 to the highest limit , σ2=σ1 under constant confining pressure, 50 and 100 kg/cm2 respectively. The whole process that the strength increases gradually to a maximum and then decreases gradually with increasing of intermediate principal stress under constant σ3 was observed . Failure and flow were effecfed not due to volume dilatancy but rather du to compression of volume under constant confining stress of 100 kg/cm 2 and a general triaxial stress state of σ3 =100 kg/cm2 , σ2=295kg/cm2 . For hard rocks , this behavior can be observed possibly only under higher temperatures and pressures. The experimental result shows that the ductility and coefficient of strain hardening are increased in proportion to the intermediate stress σ2 under constant σ3 . This is different from the behavior of hard rocks.