Abstract: The body's metabolic system has remarkably flexible ability to use a variety of dietary macronutrients as fuels. Many evidences have proved that a high-carbohydrate diet can maintain muscle glycogen level and improve exercise performance. On the other hand, some research have focused on the low-carbohydrate diet which related to “fat adaptation”. The low-carbohydrate diet can significantly increase fat oxidation and reduce carbohydrate oxidation during sub-maximum exercise, thus leading athletes to maximize fat utilization and save muscle glycogen. Theoretically, this can improve endurance performance because muscle glycogen is limited and sufficient glycogen storage is the key factor to maintain exercise ability in endurance competition. Therefore, this study summarized the research of fat adaptation with different periods in endurance exercise and high intensity exercise. In general,restricted carbohydrate diets(RCDs) can reduce high-intensity exercise performance and has potential limitations in the immune system, fatigue and muscle mass. For endurance exercise, it takes at least several months of RCDs to have the possibility of improving endurance performance without carbohydrate restoration. In addition, the short fat adaptation period will reduce endurance performance.