This paper is based on the results of in-situ and laboratory direct tests of basalt rock mass with columnar joints. Allometric curves are fitted with the test data and empirical formulas are obtained about the shear strength of the basalt rock mass with columnar joints. The analysis shows that the values of the cohesion c and the internal frictional coefficient f vary approximately with the size of test samples as power functions. The representative elementary volumes of the c and f are analyzed. The cohesive force is more sensitive to the size of samples.