The effects of short-term constraint-induced movement (CIM) therapy on the activation of the motor network were investigated with functional magnetic resonance imaging (fMRI). Movement of the less-affected arms of five patients was restricted and intensive training of the affected upper limb was performed. Functional MRI was acquired before and after two-weeks of CIM therapy. All patients showed significant improvement of motor function in their paretic limbs after CIM therapy. For three patients, new activation in the contralateral motor/premotor cortices was observed after CIM therapy. Increased activation of the ipsilateral motor cortex and SMA was observed in the other patient. Our results demonstrated that plastic changes of the motor network occurred as a neural basis of the improvement subsequent to CIM therapy following brain injury.