Abstract: Gas sensors composed of Tin Oxide（SnO₂） nanoparticles of various aspect ratios were synthesized and their aging properties were studied by absorbing current and Complex Impedance Spectrum（CIS） methods.The results indicated that devices of optimal long-term stability could be obtained by sintering the equiponderant mixture of SnO₂ nanorods and spherical nanoparticles at 850C.Using CIS method the electrical properties that are related to the microscopic structure of the samples were recorded,while the evolution of absorbing current was applied to analyze the ionic conductance that reflects the charging state of the samples.The results showed considerable fluctuation in samples’ impedance data in the early stage of aging,which then stablized after several days.The aging process presented as the change in both grain-boundary resistance and capacitance,which indicate corresponding change in the grain-boundary-barrier height and depletion-layer thickness.We suggest the DC-field-induced migration of ions within the grain boundaries as the probable cause of the phenomenon.