Thermal stress is induced by the mismatch in the thermal expansion properties of a cement sheath and casing steel, which could lead to the generation of microannulus or cracks when the temperature rapidly increases or decreases in a thermal recovery well. This stress can be relieved if the set cement has a coefficient of thermal expansion (CTE) similar to that of the casing steel. In this study, the influence of two carbonaceous admixtures (petroleum coke and gilsonite) on the thermal expansion property, compressive strength, and microstructure of the oil well cement were studied. Experimental results show that replacing the cement with different proportions of carbonaceous admixtures increases the CTE of the set cement. This effect is mainly attributed to not only the improvement in the cement microstructure as a result of the filling effect but also the thermal pressurization caused by the expansion of the volatile matter in carbonaceous admixtures and the phase transition of the gilsonite particles in the cement paste when the temperature rapidly increases. Furthermore, the transformation of the hydrate products narrows the CTE range of the hardened cement paste when the cement is subjected to a high temperature history. Although the addition of carbonaceous admixtures has a negative influence on the strength performance of the set cement, it still satisfies the needs of cementing engineering and guarantees the cementing quality.