Background. Cerebral palsy (CP), which is the most commonly encountered neuromuscular disorder of childhood, causes permanent physical deficits and sometimes intellectual deficits. Despite advances in the diagnosis and treatment of CP, the incidence of that disorder has not declined; it parallels the increased survival rates of premature infants. The children with CP may not have trunk control because they have spasticity and weakness in their trunk control. Goal. This study aimed to investigate the efficiency of functional electrical stimulation (FES) application on the abdomen-posterior back muscles in children with Cerebral palsy (CP). Materials and Methods. However 40 children with spastic CP, being treated in physical therapy, were selected by the way of random-sampling in the study, some of children were excluded by exclusion criteria and then 30 children have completed the study. The participants were randomly divided into two groups FES (n=15) and control groups (n=15). The control group received physical therapy 3 days a week in 45 minute for 6 weeks. The children in the FES group received physical therapy in addition to function electrical stimulation. FES was applied 5 days a week for 6 weeks to abdomen-posterior back muscles in 30 minute-long sessions. To evaluate the score of gross motor function measurement (GMFM) and to evaluate the trunk asymmetry in sitting, radiographic measurements were used. Result. The comparisons of the measurements of the two groups before and after the treatment showed that the GMFM standing score were statistical significantly (p<0.001) increased, and Cobb angles were decreased both groups, but the decrease in the control group was not statistically significant (p=0.128), and FES group was statistically significant (p=0.033). The comparison between groups GMFM standing score increased higher in the FES group than in the control group. Cobb angles after the treatment were statistically significant higher in the FES group than in the control group. Conclusion: To improve gross motor developing for children with CP, FES applied on abdomenposterior back muscles along with physical therapy is more effective than physical therapy alone.