Background and Aim: Cerebral palsy is a non progressive disorder and the most common movement problem in children. Many children with cerebral palsy are of hemiplegic type. One of the important problems of this children is unilateral upper limb dysfunction, that not using the affected side causes forgetfulness and more problems on this side. In this study, effectiveness of Constraint-Induced Therapy (CIT) and the maintainance of this technique on spasticity and performance of upper extremity in hemiplegic cerebral palsy children 6 to 12 years old have been investigated.Materials & Methods: In this pretest – posttest quasi experimental study, 20 cerebral palsy hemiplegic children 6 to 12 years old (15 boys and 5 girls) with 8.7 years mean age were selected. Interventions were performed about 8 weeks and patients were followed for 4 weeks. Spasticity on the affected side in wrist and elbow were tested by Modified Ashworth Scale and upper extremity function by the Box & Block test. The data were analyzed with the Wilcoxon and paired t- tests.Results: Constraint-Induced Therapy in the affected limb decreases significantly the spasticity of wrist (p =0.00) and elbow (p=0.005) and significantly increases performance score (p=0.00), and the recovery significantly maintained one month after the interventions.conclusion: Constraint-Induced Therapy of affected upper extremity could possibly decrease limb spasticity and increase function with two mechanisms: Fierstly, The reduction of not using the plegic side and secondly, neuroplasticity in CNS. This improvement may remain viable in the affected limb.