CoS2–graphene composites (CoS2–Gs) were synthesized via a facile hydrothermal reaction between cobalt ions and thiourea in the presence of graphene oxide (GO). By this means, the size and distribution of the formed CoS2 nanoparticles onto flexible graphene sheet could be effectively controlled, which is crucial for achieving of high electrocatalytic activity on I3 − reduction. Mainly due to the homogeneous single-layer immobilization of CoS2 nanoparticles onto graphene sheet with high density, CoS2–G50, which was synthesized by incorporation of 50mg GO, exhibited the highest catalytic activity and the lowest electrolyte diffusion resistance. As a result, the dye sensitized solar cell (DSSC) using CoS2–G50 as counter electrode (CE) offered a power conversion efficiency (PCE) of 6.55%, which is much higher than that of cell using sole CoS2 or graphene CE. The PCE also outperformed that of Pt CE based cell (6.20%), manifesting the application potential of CoS2–G as low cost non-Pt CE materials for DSSC.