To analyze the effects of concurrent training (CT) on the muscle fibers of Wistar rats submitted to standard and hypercaloric diets. In total, 40 rats were used, divided into 4 groups: Sedentary Group (GS); Exercise Group (GE), Obese Sedentary Group (OS) and Obese Exercise Group (OE). The animals performed a CT protocol consisting of: muscle strength training and aerobic training, carried out 3 times a week for 45 days. The smallest diameter of muscle fibers (MDF) was analyzed to evaluate muscle hypertrophy. It was observed that the OE group presented a significant decrease in MDF, compared to the OS group (OE=77.41 m m vs. OS=98.58 m m). In addition, the animals that performed CT demonstrated muscle hypertrophy (GE=74.39 m m vs. GS=72.13 m m). In conclusion, the CT with a standard diet promoted an increase in MDF while CT with a hypercaloric diet resulted in a decrease.