Abstract: The strength eharacteristics of rocks under three kinds of stress changes which probably occur in mining practice have been investigated by using a true triaxial testing machine. The strength criterion has been presented. Two rock types, Xujiahe sandstone and Jialingjiang limestone were used for all tests. It is shown by tests that the strength characteristics of rocks depond on the stress changes. Therefore, it is necessary to apply true triaxial test to study the characteristics of rock so that the stress process can be consistent with the actual stress changes of in engineering project.