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For a better understanding of gas temperature effects on plasma characteristics, a numerical study has been carried out for a dielectric barrier discharge (DBD) with a helium-oxygen mixture at atmospheric pressure. A one-dimensional time-dependent simulation code has been developed to solve continuity equations for plasma species and Poisson's equation for electric field calculation for a parallel-plate DBD reactor. To include temperature effects, gas heating by enthalpy change and Joule heating with ionic current movement are considered in the helium-oxygen plasma including 13 species reacting with one another according to 34 reactions depending on the gas temperature. Varying the ambient temperature from 300 K to 500 K, the plasma characteristics are calculated for the temporal variations and spatial distributions of electric field and species densities in the DBD region, and the different features of discharge modes are described by the voltage-current characteristic curves. A glowlike mode, which typically shows the formation of cathode fall, Faraday dark space, negative glow, and positive column in the spatial distributions of electric field and plasma density, is found in the discharge at a low ambient temperature, while a Townsend discharge mode with moderate electric field intensity and lower electron density is characterized at higher ambient temperatures. The temperature-dependent reactions strongly influence the generation and loss of species in the DBD plasma, and the decomposition of O3 into O or O2 and the quenching of metastable helium by the resultant O or O2 play an important role in determining the distinct discharge mode in the DBD of a He-O2 mixture. Furthermore, it is understood that the discharge-mode transition is controllable by the coupled effects of oxygen additive concentration, frequency, and gas temperature. A small amount of O2 additive or a high-frequency operation exhibits a glow- - mode in a specific range of ambient temperature, of which reason can be explained by density variation and quenching of helium metastable species caused by the produced oxygen-related species.