The aim of this study was to investigate the effects of a school-based cardiovascular-fitness-training program in children with mental retardation (MR). Thirty boys (8–15 years old) with mild to moderate mental retardation were randomly divided into 2 groups—experimental (EX) and control (CN). The EX group underwent 10 weeks of training 3 times/week for a duration of 1 hr/session at 60–80% peak heart rate. At Week 10, significant increases in 20-m shuttle-run-test (20-MST) laps were observed for the EX group. No improvements were found in percent body fat. No changes were observed for the CN. The school-based training program might prove useful in improving the cardiovascular fitness of children with MR.