The propagation and coalescence of microcracks of a fine-grained marble after treatment under different high temperature cycles is observed and analyzed using a polarizing microscopy in this study. The thermal cracking behavior in the tested rock specimen is investigated by statistically analyzing the length

width

and quantity of the generated microcracks. The main results obtained from this study are as follows: (1) The number and the total length of thermally-induced microcracks inside the rock specimens after different numbers of thermal cycles are disparate. For specimens without thermal treatment

the grains are compacted and cemented well with each other

and no obvious microcracks are observed. With four thermal cycles’ treatment

the microcracks are generally observed in grain boundaries. The width and length are significantly increased as compared with those of specimens without thermal treatment. The width of micro-cracks in rock specimen with sixteen thermal cycles’ treatment reaches 55 μm

which is about four times as compared with that in rock specimen with four thermal cycles’ treatment. (2) The anisotropy of thermally-induced microcracks inside the rock specimens after different thermal cycles is also analyzed. The results show that there is no obvious preferred direction in the thermally-induced microcracks. It also shows that the sampling position of thin sections has negligible effect on the number of the thermally-induced microcracks. (3) The linear crack density is also found to increase as the number of thermal cycle increases. The linear crack density in rock specimen with sixteen thermal cycles’ treatment is about 21 times when compared with that of rock specimens without thermal treatment.