The adsorption of molecules (NH3, NO, N-2, H-2, CO2, CH4, CO, HCN, SO2, O-2, NO2, and H2S) on C2N monolayer was investigated by using density functional theory calculations to exploit the potential gas sensing application of C2N monolayer at NH3 and NO detection. NH3 and NO are adsorbed on C2N monolayer with apparent adsorption energy and charge transfer. The semiconducting behaviors of C2N monolayer are transformed into conducting behaviors due to NH3 and NO adsorption, indicating there is a significant change in electrical conductivity of C2N monolayer. However, the weak physisorption of N-2, H-2, CO2, CH4, CO, HCN, and O-2 on C2N monolayer is characterized by very small adsorption energy and inconspicuous charge transfer, and hardly changes the electrical conductivity of C2N monolayer. The adsorption of NO2, SO2, and H2S on C2N monolayer is too strong to make their desorption very difficult, which hinders the reuse of C2N sensor, however, the recovery time of NH3 and NO at room temperature is predicted to be 1.2 ms and 23 mu s. Thus, C2N monolayer is a promising candidate for molecular sensor for NO and NH3 detection for future experimental validation.