Japanese macaques have the species-specific communication sound called "coo call" for locating group members and maintaining within group contact. Monkeys could discriminate individuality only by listening to the voices, but it has been still debated how the fundamental frequencies (F0) and filter characteristics of vocal-tract properties (VT) contributed for discriminating individuals in nonhuman primates. The purpose of this study was to investigate the acoustical key for monkeys to discriminate individuals who made coo calls in Japanese macaques. Two animals were trained with standard Go/NoGo operant conditioning to discriminate coo calls of Monkey A (cooA) and Monkey B (cooB). Both cooA and cooB were recorded from unfamiliar monkeys. While calls from the same monkey were presented (NoGo trial), subjects had to keep depressing a lever. When the stimulus were changed from one monkey to another (Go trial), subjects had to release the lever within 800 ms from the offset of the stimulus. Test stimuli were synthesized by combining F0 and VT from the other individual (F0cooA-VTcooB was synthesized from the F0 of cooA and VT of cooB, while F0cooB-VTcooA was created from F0 of cooB with VT of cooA). A test stimulus was presented after cooB was repeated 3-7 times, and each type of test stimuli was played for 6 times. Data showed that monkeys responded F0cooA-VTcooB similar to the response for cooB, while animals responded to F0cooB-VTcooA like the response to cooA. Our results suggested that VT was important to discriminate individuals who made coo calls in Japanese macaques.