To study whether consumed dietary fat has a linear relationship or a threshold with glycemic controls, female C57BL/6J mice were fed different levels of a safflower oil (10, 20, 30, 40, 50, and 60% of total energy) diet ad libitum for 15 wk. Food intake, body weight, parametrial white adipose tissue (WAT) and liver weight were measured, and oral glucose tolerance tests were conducted. Although there was no significant difference in average energy intake, graded increments of safflower oil resulted in graded deterioration of glucose tolerance during 5 and 12-wk feeding, and deterioration of glucose tolerance was more manifested after 12-wk feeding as compared to 5-wk feeding. After 12-wk feeding, a significant deterioration of glucose tolerance was observed in diets of more than 40% fat. Graded increments of body weight and WAT weight were observed, and their weight increases were manifested in diets of more than 30% fat. These data indicated that the amount of dietary fat had an almost linear relationship with glucose tolerance, and significant differences were observed in mice fed diets more of than 40% fat.