The effects of annealing on gas sensing properties of NOx gas sensor based on novel material ZnGa2O4 epi-layer grown by MOCVD were studied. The metal-semiconductor-metal (MSM) structure sensor with Ti/Al/Ni (50/75/25 nm) electrode in the multilayers which were deposited by an E-gun evaporator and patterned by a lift-off process. The devices were annealed at 700 ºC in N2 ambient for 1hr, and the sensing area is 30um x 250um. The results show that the sensitivity of the ZnGa2O4 gas sensor increases and the response time reduces after annealing. The sensitivity is defined as Rg/Ra, where Rg is the resistance with analyzed gases, and Ra is the resistance with the dry air. At the operation temperature 300ºC, the sensitivity of sensors without thermal treatment are 1.026, 1.015, 1.009, 1.003, and 1 when exposed to NO concentration 6.25ppm, 1ppm, 500ppb, 250ppb, and 125ppb, respectively. After 700ºC annealing for 1hr, the sensitivity remarkably increases to 52.108, 10.491, 7.744, 4.961, and 3.942 with the same NO concentration as mentioned above. Not only the sensitivity increases more than 10 times but thin-film can detect extremely low NO concentration (125ppb) after thermal treatment. The sensitivity is linear dependent on the NO concentration. Besides, the response time improved all under 30s with the concentration range from 1ppm to 125ppb. Most important of all, the sensors show excellent selectivity which means the sensitivity were all below 1.02 when exposed to CO, CO2, SO2 gases with 1ppm. The results point out that the ZnGa2O4 gas sensors after annealing exhibit the better NO sensing properties, shorter response time and outstanding selectivity.