A gas dynamic method, to estimate the mixed-mean gas temperature in a stagnation chamber of a cold spray nozzle, is proposed. The method does not use thermocouple but mass flow rate of the process/carrier gases and stagnation pressure upstream of the nozzle throat. The method has been validated to have measurement accuracy within 3% for air, 5% for helium for the heated process-gas flow fully mixed with the room-temperature carrier gas at the nozzle throat. In the present study, the method was applied to the heated process-gas flow partly, not fully, mixed with the room-temperature carrier gas at the nozzle throat. The mixed-mean gas temperatures estimated by the present method were validated by comparing them with measured values using a coiled sheath-type φ 0.5mm thermocouple, inserted in the stagnation chamber of the nozzle. The gas flow experiment of cold spray was conducted under the conditions of gas pressure up to 0.96MPa, and heater-exit gas temperature up to 673K. The working gases selected were air and helium. The species of the process gas and the carrier gas was the same. The ratios of the mass flow rate of the carrier gas against that of the total mass flow rate selected were 0.20 and 0.33 for air, 0.20 for helium as working gas. The comparison of the estimated and measured mixed-mean gas temperatures shows that the estimation error of the present method is less than 2.5% for air, and less than 4% for helium gas.