To investigate whether long-term ingestion of guava juice has anti-diabetes and/or anti-obese actions, we employed spontaneous non-insulin-dependent diabetes mellitus Otsuka Long-Evans Tokushima Fatty (OLETF) rats and its control strain Long-Evans Tokushima Otsuka (LETO) rats. Thirty rats of each strain were divided into three groups consisting of glucose, vitamin E, and guava juice ingestion groups. Ingestion of these test solutions was continued from 9 to 32 weeks old. Serum lipid parameters including total cholesterol, triglyceride, free fatty acid, and high-density lipoprotein (HDL)-cholesterol were measured. Oral glucose tolerance test (OGTT) was performed at 32 weeks old rats and at 42 weeks old rats (10 weeks after discontinue of ingestion of guava juice), and then blood glucose levels and plasma insulin concentrations were measured. There were no significant differences in body weight, the amount of food intake and the volume of drink among the groups in OLETF rats. Although the blood glucose level in the guava juice group was not changed as compared with the glucose group, the amount of initial insulin secretion was significantly increased in OLETF rats and was restored by discontinue of ingestion of guava juice. Therefore, the long-term ingestion of guava juice may increase plasma insulin concentration in OLETF rats.