The obscurity in selectivity mechanism has resulted in dilemmas in the research of selectivity properties of metal oxide semiconductor (MOS) gas sensing materials. In this work, selectivity mechanism of ZnO nanorod arrays to CH3OH is revealed by systematically simulating response and recovery reactions of three gas molecules (CH3OH, NH3 and CO) based on density functional theory (DFT). Results suggest that selectivity to CH3OH is due to three factors: i. Response reaction of CH3OH on ZnO (1 0 (1) over bar0) is most exothermic, which is main reason for the selectivity against NH3. ii. Intermediates as well as products oxidized from CH3OH have strongest absorption ability, which hinders reabsorption of O-2 molecule, mainly leading to selectivity against CO. iii. H adatoms dissociated from CH3OH transfer most electrons to ZnO surface layers, which is also responsible for the selectivity. This work provides a new insight into selectivity mechanism and a practicable method to predict selectivity properties of MOSs.