Abstract: A series of uniaxial compression test, Brazilian test, crushing compression test and point load test on coal samples from Zhangcun mine 3# coal seam were carried out. The results show that the dispersion degree increases by the following order: Brazilian test, uniaxial compression test, and point load test. After the irregular samples compared to the regular samples，the dispersion degree of point load test is obviously too large. Among all those regular samples, the point load strengths are roughly the same by parallel bedding and vertical bedding loading methods. There is a correlation among the Protodyakonov's coefficient, uniaxial compression strength, tensile strength and point load strength of the coal samples, which could obtains relevant parameters by their interconversion, and the conversion strength can be used as a reference of coal classification strength index.

Key words:  rock mechanics, coal sample, point load test, strength, conversion coefficient, dispersion coefficient