Abstract: A micro-model of thermal resistance change with fiber breakage was developed to describe the correlation between the thermal resistance change and the fiber damage in unidirectional fibers reinforced composites, and a qualitative analysis of the thermal resistance change was carried out based on the present model. Heat transfers in longitudinal and thickness directions of the composite were both analyzed. Analytical solutions were derived for the thermal resistance change in both longitudinal and thickness directions of the composite by the standard Weibull model in combination with the failure length of fibers as the minimum length of the fiber segments. The process of fiber break under applied tensile stress was simulated with Monte-Carlo random method, during which the thermal resistance change in the composite was calculated simultaneously. The results of the present study show that the thermal resistance changes in both the longitudinal and thickness directions increase linearly with the number of fiber breakage in the composite. For larger fiber volume fraction, the thermal resistance changes more greatly. The thermal resistance change in longitudinal direction of the composite increases greatly by the fiber/matrix thermal conductivity ratio β, but the increase amplitude reduces gradually while the fiber/matrix thermal conductivity ratio greater than 10 (i.e. β>10). However, the thermal resistance change in thickness direction increases dramatically from the beginning and then decreases with the increase of the fiber/matrix thermal conductivity ratio β, and the maximum value of the thermal resistance change occurs for the fiber thermal conductivity close to that of the matrix (i.e. around β=1).