Fractography was used in order to investigate the effects of Ta addition on the diffusion behavior of carbon in molybdenum. Mo-1.5mass%Ta alloy was carburized by the solid-state carburization process at a temperature between 1073 and 1473 K. Fracture surface of the specimen was observed before and after the carburization process and carbon diffusion distance was estimated from the change in fracture mode as a function of distance from the specimen surface. Results are as follows. (1) Ta addition slightly decreased the carbon diffusion distance. (2) Ta addition had almost no effect on the activation energy for diffusion. (3) These results suggest preferential combination between carbon atoms and Ta atoms is only minor.