Abstract: Based on the discussion properties of brittle failure of frozen soil, requirements on dimension of sample for the definition of fracture toughness under linear elastic and plane strain condition are analyzed in this paper. Testing methods, which include preparation of samples, determination of the conditional load P- q  and other details in the test, are explained in detail. The relations between K- Ⅰc (K- q ) and temperature, water content and loading rate are also analyzed respectively on the basis of the testing result of frozen clay.