Abstract: It is difficult to eliminate the effects of stress release and soil disturbance when using conventional test method to determine the ratio of over consolidation (OCR) of clays. A new determination method of clay OCR by lateral pressure test using cavity expansion and critical state concepts is presented, which can make full use of lateral pressure test results and critical state parameters. The new method is used to estimate OCR of 4 types of clays and the results are compared with those values obtained from improved consolidation test. The results show that for over consolidated clays, the OCR obtained by the new method is in agreement with that of consolidation test, and the OCR obtained by the new method is slightly larger than that of consolidation test for lightly over consolidated clays, which all meet the requirements of engineering survey. As a new in-situ method to evaluate OCR of clays, the new method expands the application scope of the lateral pressure test and has certain engineering application value.