In four-cycle single-cylinder motorcycle engines, high Hertzian stress is generated on and beneath the big-end surface of the connecting rod. If the surface strength would be improved, the diameter of the big-end could be made smaller, making the entire engine smaller and lighter. Therefore, application of carbide dispersion carbonitriding using a vacuum furnace (hereinafter referred to as “vacuum CD carbonitriding”) on the big-end surface was investigated. Vacuum CD carbonitriding was carried out by three processes. The first was a CD carburizing process. This process is done to obtain granular cementite, but in order to avoid decreasing the strength, it is necessary to prevent the formation of coarsened cementite at the grain boundary. The second process was a refining process. This process is done for the purpose of refining the prior austenite grain size. The third process was a carbonitriding process. This process is done for the purpose of increasing hardness in the vicinity of the surface. The strength of vacuum CD carbonitrided specimen was evaluated using a ball-on-disk type rolling-contact-fatigue testing machine. Vacuum CD carbonitriding has six times as long lifetime as conventional carburizing has. Vacuum CD carbonitrided connecting rods have been used in single-cylinder motorcycle engines since 2011.