A middle size triaxial test apparatus was developed, to measure deformation and strength of a weathered soft rock. The undisturbed samples with high quality were taken from a dam site by using a socalled core drilling method. In order to avoid the damage of soft rock structure due to saturation of specimens for measuring volume change through burette, the lateral deformation of specimens was directly measured in unsaturated condition by using three rings. Using the developed triaxial test apparatus, isotropic compression tests and consolidated-drained triaxial compression tests were performed on unsaturated or saturated undisturbed samples under confining pressures of 98kPa, 196kPa and 392kPa. The test results show that the tested weathered soft rock under both unsaturated and saturated conditions has strong confining-pressure dependency of deformation. Comparison of results of triaxial tests on unsaturated and saturated specimens shows the samples become somewhat weak being saturated. Cohesion becomes small while the internal friction angle keeps constant when the sample is saturated.