Introduction: A number of studies have shown that young children’s participation in physical activity (PA) and sedentary behavior (SB) would affect health in later life. Therefore, this systematic review aims to determine the relationship between physical activity, BMI and sedentary behavior in young children. Methods: We searched English language articles from databases of ERIC, ProQuest, PubMed and Web of Science related articles as well as the reference lists of existing articles between 2008 and March 2018 to retrieve eligible studies. The studies that assessed the relationship between physical activity, sedentary behavior, and BMI in young children. Results: In all, 43 studies met the inclusion criteria and were synthesized in the review. Most studies showed that male children participate in physical activity more than female children. Younger children participate in physical activities more often than older children. There were some studies showed children with normal weight participate in sports activities have more time than overweight children. Sedentary behavior (SB) is increasing, while physical activity (PA) is decreasing in young children. There is a relationship between children’s sedentary behavior and their participation in the physical activity that decreasing any type of sedentary behavior is associated with more physical activity in young children. Conclusion: As far as we know, this systematic review synthesizes the existing results about the relationship between physical activity, BMI and sedentary behavior in young children. In conclusion, measuring children’s sedentary is good for their growth and encourage children to take more physical activity whatever in school or at home. That will reduce the chance of a child suffering from chronic diseases when they grow up.