The behavior of an interface crack between the matrix and the fiber in composite materials subjected to transient heating and cooling is studied. The differences in mechanical and thermal properties of fiber and matrix lead to the failure of the interface between these two constituents. A cylindrical cell of fiber and matrix was analyzed using a cylindrical crack at the interface. The cell was subjected to a transient thermal load, and the corresponding transient strain energy release rate G was calculated using the finite-element method. When the matrix boundary was unconstrained, the largest G occurred during the transient cooling and subsequent heating and was mostly due to shearing of the interface. Constraining the matrix reversed this trend. When the matrix was free, reducing a*m/a*fand E*m/E*fgenerally reduced G, but equality of fiber-matrix thermal expansion coefficients did not always result in the smallest G value. The increase in the fiber volume ratio was detrimental during transient thermal loads but was beneficial for steady-state temperature excursions. The assumption of the free matrix boundary versus the case of a restrained boundary has a considerable effect on the behavior of the fiber matrix interface. Finally, the initial stress free temperature was shown to have a significant effect on the behavior of the fiber-matrix interface crack.