Abstract: In order to reveal the influence of mechanical properties of coal seam and its roof and floor rocks on crack extension during hydraulic fracturing, rock three-axis stress was used to test rock mechanical properties of various coal seams and their roof, floor rocks in the Hancheng area, Shaanxi Province. Characteristics and differences of mechanical properties of various rocks are summarized in this study. Through analyzing the relationships between porosity and water content, we explored influences of the differences of mechanical properties on fracture extension. The testing results show that rock strength decreases with increasing porosity, rock softening property depends on mineral compositions and rock porosity, and water contents greatly affect the strength of various rocks in different ways. For a shallowly-buried coal seam, roof and floor rocks of which have significant differences in elastic modulus, hydraulic cracking can be easily restricted within coal seam but extends much further.