In this paper, the influence of size effect on the sensitivity of Cu/CuNi thin-film thermocouple was studied through experiments and theoretical analysis. Cu/CuNi thin-film thermocouples with different thicknesses of 0.5μm, 1.0μm, 1.5μm and 2.0μm were fabricated on the substrate with SiO 2 lining by means of reactive magnetron sputtering. After static calibration, the sensitivities of the thermocouples were obtained respectively of 46.49μV/°C, 45.23μ/°C, 44.56μV/°C and 43.94μV/°C. According to the experimental results, it can be seen that the sensitivity of Cu/CuNi thin-film thermocouple is higher than that of the bulk Cu/CuNi thermocouple, and the sensitivity S will increase with the reciprocal of the thickness 1/δ as long as the thickness of the film is greater than the critical value.