Improving the understanding of catalyst characteristics and the ability to design new catalytic materials is the key to overcoming the application limitations of semiconductor metal oxide (SMO)-based gas sensors related to relatively low sensitivity and selectivity. Nowadays, the important role of twin defects of metal nanocrystals (NCs) in enhancing the gas sensitivity of SMO is largely unrevealed. Herein, we show a simple synthesis strategy to selectively prepare multiply twinned PtCu icosahedra NCs and single crystalline PtCu octahedra NCs in a controlled manner, which have similar chemical composition, particle size, work function and the same exposed crystal faces. They provide an ideal platform to study the effect of twin defects on gas sensitivity. The gas sensing test showed that PtCu NCs supported on SnO2 display the interesting crystalline nature-dependent acetone sensitivity. PtCu icosahedra-SnO2 exhibits a higher response and better selectivity to acetone than PtCu octahedra-SnO2 under high humidity conditions due to the presence of high density of twin defects and surface strain. This work highlights the uniqueness of twin defects in enhancing the gas sensitivity of metal NCs