A series of Graphene-ZnGa2O4composites (G-ZnGa2O4) were synthesized by hydrothermal method. The prepared materials were characterized by means of XRD, SEM, TEM, Raman and XPS, respectively. The gas sensing properties of G-ZnGa2O4 composites were investigated. The results indicated that the graphene content has a great influence on the response and the gas sensing selectivity, the optimal composition of G-ZnGa2O4 gas sensing material was 0.1%G- ZnGa2O4. The sensor based on 0.1% G-ZnGa2O4 exhibited high response to 1000 ppm formaldehyde when operated at 203oC and the response reached 32.2; the detection limit for formaldehyde was as low as 1ppm; the gas sensing selectivity to formaldehyde was also good, the ratio of S1000 ppm formaldehyde and S1000 ppm acetone reached 26.8. The response time and recovery time for 1000 ppm formaldehyde were 11 s and 5 s, the response time and recovery time for 1 ppm formaldehyde were 6 s and 5 s, respectively.