Novel impedance-type humidity sensors were made by coating diamine-functionalized graphene oxide (GO) films on alumina or plastic substrates. Ethylenediamine (EA) and 1,6-hexanediamine (HA) were used to functionalize GO using N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) (EDC/NHS) as the coupling reagent. The effects of the chain length of diamine on the electrical and humidity-sensing properties of the diamine-functionalized GO film were investigated. The diamine-functionalized GO film was characterized by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). The impedance-type humidity sensor that was made of the EA-GO film exhibited a wide range of working humidities, a high sensitivity, satisfactory linearity, a small hysteresis, high flexibility, a short response/recovery time, a weak dependence on temperature and high long-term stability. The linearity of the humidity sensor depended on the applied frequency. The ions (H3O+) dominate the conductance of the diamine-functionalized GO film.