Abstract: The tensile strength of rock is the ultimate stress of the rock cementation during its destruction.Brazilian test should be done to determine the tensile strength of rock.However,various tests have indicated that,Brazilian test can not exactly reflect the tensile strength of rock,and there are some errors in the test results.Through Brazilian test of hard rock,the relation ship between the error and the thick diameter ratio was obtained.Moreover,the major influence on error came from the variation of the thickness of test samples.The error was the smallest when the thickness-diameter ratio(L/D) was close to 1.0.Therefore,It was suggested that ther L/Dratio of test samples should be 1.0 strictly.