The mechanical properties of coal are the basic data for the design of drilling completion programs

coal bed methane development plans and construction methods; and the important factors which influencing the effect of reservoir reconstruction. The mechanical parameters of coal under different stress conditions are significantly changed caused by the existence of large plenty of endogenous fractures

structural fractures and micropores. At present

researches on the changing rules of the effective mechanical parameters of coal under different stress conditions

and the internal causes for those changing rules are relatively lacking. By triaxial compression tests

the characteristics of effective elastic mechanical parameters of medium rank coal under different stress conditions are studied. The results show that the deformation of coal under different stress conditions is significantly affected by the pore and fracture. In the elastic deformation stage

the effective elastic modulus and effective Poisson's ratio of medium rank coal increase with the increase of confining pressure. The effective elastic modulus and effective Poisson's ratio of coal present a nonlinear relationship with confining pressure respectively; and their values tend to be constant when the confining pressure increases to a certain degree. Based on the triaxial compression test

the effect mechanism of stress conditions on the effective elastic properties of coal is analyzed; empirical formulas of the effective elastic modulus and effective Poisson's ratio are given for medium rank coal with extensional fractures in it. Meanwhile

the applicability of the formulas is analyzed.