Mental Retardation (MR) has negative effects on a child’s physical fitness as well as his or her social and cognitive abilities during the life. Research indicates that children with MR have a poor level of physical fitness compared to their nondisabled peers (Pitetti et al., 2001). The effects of swimming and aquatic exercises on physical fitness and well being have been recognized for people with disabilities (Lepore, 2000). Moreover, aquatic exercises are highly recommended for children as a recreational activity (Sherrill, 1986). Although there have been studies about the effects of aquatic exercises and swimming on children without disabilities, there had been no research especially for the effects these exercises on children with MR. Therefore, the purpose of this study was to determine the effects of a 10-week swimming program on the respiratory functions of children with MR. The participants were nine trainable and seven educable children with MR in a rehabilitation center. The mean age of groups were 12.22±0.49 years in trainable group (n = 9) and 14.71±0.52 years in educable group (n = 7).The training program was applied for 10-week, two times weekly for 40 minute each session. Pre and post tests measurements were taken in FVC, FEV1, MEF%50, PEF, MVV, VCMax, and FEV1/VC parameters using a VICA TEST P2 spirometer. The Mann Whitney U test was used to compare the two groups and the Wilcoxon Matched Pairs Signed-Rank test was used within each group. The results of this study indicated that there were no significant differences between the groups (p>0.05) exception of FEV1/VC parameter in the favor of educable group (p<0.05). However, this study found that only educable group improved significantly within the group (p<0.05) in FEV1, MVV, and VCMax parameters. Trainable group did not improve within the group (p>0.05) in all parameters. The results of this study show that 10-week swimming program had no effect on all respiratory functions of trainable group. In addition, swimming program had only significant effect on certain parameters of educable group. This could be related to physiological changes in muscle endurance as a result of training regimen and also longer period of time is needed to have a significant effect on respiratory functions of children with MR. This study has several limitations. Therefore, future studies should address the effects of different physical activities on the respiratory functions of children with MR.