Graphene nanoplatelets were activated by a hydrothermal method using potassium hydroxide (KOH) as an activating agent. The activation process significantly increases the sample specific surface area from 2.4 m2/g to 48 m2/g. The activated graphene nanoplatelets (aGNP) were used as counter electrode (CE) in dye-sensitized solar cells (DSSCs). DSSCs fabricated using aGNP as counter electrodes were tested under standard AM 1.5 illumination with an intensity of 91.5 mW/cm2. The optimized device achieved an overall power conversion efficiency of 7.7%, which is comparable to the conventional platinum counter electrode (8%). The device efficiency (η) and short-circuit current density (Jsc) are dependent on the thickness which is in turn proportional to the surface area, indicating that the surface area could be a dominating factor that dictates the device performance. In addition, it was found that the performance of DSSC is sensitive to the thickness of aGNP thickness. Electrochemical impedance spectroscopy (EIS) indicated that aGNP possessed excellent electrocatalytic activity for triiodide reduction at the interface between electrolyte and counter electrode. This suggests that the aGNP is a promising, cost effective counter electrode material with similar efficiency to Pt counter electrode for the DSSC application.