In the T2 distribution of carbonate rocks, an anomaly is recognized that the short relaxation in the irreducible water condition is more prominent than that in the saturated water condition. By reviewing the basic theories of NMR applied in the field of porous media, diffusion coupling effect is discussed. According to some samples with different porosity and permeability in Upper Cretaceous carbonate rock, Iraq, a series experiments are conducted to study the relation between the pore structure and T2 distributions in both saturated water state and irreducible water state, and the problems in the characterization of pore structure and the calculation of irreducible water saturation by the method of T2 cutoff value and spectral coefficient respectively. It is shown that the T2 anomaly is common in carbonate rocks with good pore connectivity, where the diffusion of fluid molecules in the macro- micro pores under saturated water condition leads to the homogenization of hydrogen proton distribution in a porous medium and the T2's average distribution (diffusion coupling effect). Thus the T2 distribution of carbonate rocks in saturated water state would characterize the connectivity between pores rather than the pore size. To calculate the irreducible water saturation, the T2 cutoff value is ineffective due to the diffusion coupling effect, and the method of spectral coefficient from Coates gave an empirical equation, in which the spectral coefficients including m and b should be corrected according to the actual differential pressure.