A Physique index was devised for evaluation of fatness in Japanese macaques. Parameters for a function of the physique index were statistically obtained from body weight and trunk length of immature monkeys of Primate Research Institute, Kyoto University. Index-male is expressed as 1.01 ^*[Body weight (Kg)]^* [Anterior trunk length]^-2.3* 10⁷, and Index-female is 1.05^*[Body weight]^*[Anterior trunk length]^-2.3* 10⁷. The indices show an age change pattern common to the both sexes; an infants index which starts at 100 decreases with age during juvenile period till four years of age, and then rapidly increases to 120 in males and to 110 in females. The index of males keeps gradual increase, but that of females neither increases nor decreases. Application of physique index to various Japanese monkey groups living in different environments, shows that monkeys in the northern and heavy-snow falling area have a larger index (130-140), and those in the warmer area have a smaller index (ca. 90-100). The larger index means that they show adaptability for winter by means of preservation of energy. Influence of food control is so strong as to change the physique index widely.