Abstract: The influence of freezing and thawing cycles on the physical and mechanical properties of granites with coarse and fine grains was studied by freezing and thawing cycle experiment and rock mechanic test. Rock samples were detected by nuclear magnetic resonance (NMR), their transverse relaxation time spectra and NMR images were got before and after freezing and thawing cycle experiment, and the porosity variation, pore structure and evolution laws were discussed. The macro phenomenological theory and the self-consistency theory were used to analyze the degradation principle of rock samples in macro and micro scales, respectively. It is found that the number of pores increases in the action of freezing and thawing cycles, which weakens the strength of rock samples. The theoretical values of damage models are in consistence with experimental data, but different damage theories show different results about the development of damage tendency. Granite with fine grains shows a better durability in the action of freezing and thawing cycles.