A SnCl2 solution was used to prepare SnO2-coated multi-walled carbon nanotubes (SnO2/MWCNTs), and their hydrogen sensing properties at room temperature was studied. SEM and TEM observations indicate that SnO2 nanoparticles with a size of 5 nm are uniformly coated on the MWCNTs, forming a continuous SnO2 coating. The SnO2/MWCNTs are sensitive to 10-4 hydrogen at room temperature. During the sensing measurement at room temperature for 10-3 hydrogen mixed with argon gas, the current change with gas concentration is reversed after the mixed gas is switched to air. This can be ascribed to the reaction between O2 in air and residual H2 around the SnO2/MWCNTs to form adsorbed H2O and the subsequent desorption of H2O by substitution adsorption of O2 in air. The H2O adsorption may lead to a decrease of the electric resistance of SnO2/MWCNTs and thus an increase of current measured. The reversal of current implies that the SnO2/MWCNTs may possess a good sensitivity to humidity at room temperature.