The effect of microwave heating on the fractal dimension of coal cores is evaluated by an experimental work, which is carried out by nuclear magnetic resonance. This paper verifies that the membrane-bound water model is appropriate for explaining the state of bound-water in pores of coal samples. The paper also verifies fractal dimension as an intrinsic property of a surface that does not depend on porosity. The pore size, pore number, connectivity, and porosity increased after microwave irradiation. Furthermore, a theoretical equation was proposed to calculate fractal dimensions based on the transverse relaxation time (T-2). The fractal dimensions of coal cores decreased after microwave treatment along with the permeability increase. These results suggest that microwave irradiation is a potential method for degassing coal seams in the future.