The mechanical properties of unsaturated soil become complicated because of the pore air. The pore air affects the strength and the deformation of unsaturated soil. A series of triaxial compression tests was performed on samples of Yunnan red cohesive soil with different initial saturation degrees and different dry bulk densities. The pore air can influence the strength and deformation of unsaturated soil. The cohesion c does not decrease monotonously with increasing saturation. The friction angle decreases with increasing saturation degree. Both cohesion c and friction angle increase with increasing dry bulk density. The dilatation occurs frequently in the process of unventilated testing. As increasing saturation degree and decreasing dry bulk density

the dilatation disappears. The destructional forms of samples may be influenced by dilatation.