In-Situ composite materials of graphene (Gr) and ZnO microspindle with different Gr contents (1, 2 and 5 wt.%) were prepared via a facile one-step hydrothermal route with the assistance of hexamethylenetetramine. Graphene does not affect the hexagonal wurtzite crystal structure of composite materials but strongly affects the morphological, structural, optical and photocatalytic properties of composite materials. In detail, Gr causes a decrease in both the dimensions of ZnO microspindle and photoluminescence efficiency. The average crystalline size and microstrain first increase from 28.9 nm and 0.00394 to 49.5 nm and 0.00524 when Gr content increases from 0 to 1 wt.%, respectively; then decrease to 39.6 nm and 0.00404 when Gr content increase from 1 to 5 wt.%, respectively. Both pristine and composite materials show high photocatalytic activity with high methylene blue degradation efficiency?more than 90%-just after 40 min under UV irradiation. Composite material having a Gr content of 5 wt.% shows the highest degradation efficiency of 96.5% which confirms the role of Gr in enhancing the photocatalytic activity of ZnO.