This study is a randomized double-blinded trial investigating the effect of Transcranial Direct Current stimulation (tDCS) on upper limb motor function rehabilitation in chronic stroke patients. 51 subjects will be recruited from National University Hospital (NUH) and be randomized to receive one of the followings: * Group 1 will receive 1 mA anodal tDCS stimulation to the ipsilesional M1 of cortical representation of the affected upper limb; * Group 2 to receive 1mA anodal tDCS to the contralesional premotor cortex; * Group 3 to receive sham tDCS stimulation with anode placed over the scalp area corresponding to ipsilesional M1. tDCS will be performed once a day together with standardized occupational therapy (GRASP) for 20 sessions within 30 days. Group 1 and Group 2 will receive tDCS for 20 minutes during each session, while Group 3 only receives the current stimulation for 20 seconds. GRASP will be performed daily together with tDCS, either concurrently with or immediately after tDCS stimulation. The outcome measures will be measured at baseline, after intervention and 1 month after intervention, including: 1. TMS measurement of corticospinal excitability; 2. functional MRI scan; 3. High density EEG (HD-EEG) evaluation; 4. Clinical measures on upper limb motor function; 5. Cognitive tests.