Objective: The aim of the present study was to examine the effect of relaxation training on reducing of anxiety and depression in mothers of children with intellectual disabilities.  Method: The study was a quasi-experimental research with two heterogeneous groups design with pretest and posttest. First, 20 mothers who were regularly able to participate in the classes of relaxation training were selected as experimental group and then, 20 mothers were also selected as control group via parameters such as the rate of anxiety and depression, literacy level, educational level, and severity of intellectual disabilities in the child. The group exercises of relaxation training have been conducted for experimental group three times a week for 2 months. Data of mothers’ anxiety and depression in both groups were obtained by Beck Anxiety Inventory and Beck Depression Inventory-II in two stages of pretest and posttest. The data were analyzed with covariance analysis. Results: The findings indicated that relaxation training exercises were effective in decreasing symptoms of both anxiety (P<0.001) and depression (P<0.001) of mothers. Conclusion: Generally, the results indicated that the short-term relaxation training technique designed in this research could reduce anxiety and depression of mentally retarded children's mothers participated in this study.