In this paper, different contents of molybdenum oxide/tungsten oxide (MoO3/WO3) composite nanostructures were synthesized by hydrothermal method. Field emission scanning electron microscopy images revealed that the morphologies of WO3 nanostructures were significantly influenced by the Mo amount. Furthermore, the introduction strategy of MoO3 into WO3 could effectively improve the gas sensing properties. Especially, the sensor based on the 4 mol% MoO3/WO3 composite nanostructures exhibited enhanced gas sensing performance, giving a low limit of detection (500 ppb). It shows high responses of 28.5 and 18.2-100 ppm ethanol and acetone at the operating temperature of 320 A degrees C, which were about 2.3 and 1.7 times higher than those of the pure WO3, respectively. The enhanced sensing properties of MoO3/WO3 gas sensor can be attributed to the addition of MoO3, which has been discussed in relation to the gas sensing mechanism.