Abstract: Objective: This study was conducted to develop an animal model of obesity by monitoring the phenotypes of long-tailed monkeys (Macaca fascicularis) fed a high energy diet. Methodology: Fifteen adult male Long-Tailed Monkeys (LTM) weighing between 4 and 5 kg were divided into three groups of five that received one of three diets: Diet A: A commercial monkey chow diet, Diet B: Monkey chow supplemented with egg yolk and Diet C: In which wheat was the main ingredient and beef tallow was added. Body measurements, including body weight were collected monthly. Results: The LTM fed Diet B (Egg yolk-supplemented) consumed more food compared to those fed Diet A and C. Animals in the Diet B group had significantly (p<0.01) larger waist circumference, hip circumference, chest circumference and skin fold thickness compared to the other two groups. There was a close relationship between body weight and waist circumference (r = 0.863), chest circumference (r = 0.889) and hip circumference (r = 0.890). Based on the main component analysis, LTM fed Diet B had higher rates of obesity compared to LTM fed Diet A or C. Conclusion: Body characteristics of LTM fed different diets can be used to determine the presence of obesity. The LTM can serve as a useful animal model of obesity and findings from this model can be applied to studies of obesity in humans.