We discuss a merit function F as judgment of photo-thermal conversion efficiency instead of two independent parameters: solar absorptance \alpha and thermal emittance \varepsilon. The merit function F is developed using Essential Macleod software to optimize the photo-thermal conversion efficiency of solar selective coating. Bruggeman and Maxwell-Garnett models are used to calculate the dielectric function of composite cermet film. Mo, W, V, and Pd are used as metallic component as well as infrared (IR) reflector materials, and SiO2, Al2O3, AlN, and TiO2 are used for dielectric component or antireflection (AR) layer materials. The layer structure can be described as substrate (Sub)/IR reflector/ high-metal-volume fraction (HMVF)/low-metal-volume fraction (LMVF)/AR. Results show that Mo-Al2O3, Mo-AlN, W-SiO2, W-Al2O3, VSiO2, and V-Al2O3 double-cermet coatings have high conversion efficiency which is greater than 86%. The best among above is Mo-SiO2 with \alpha =0.94, \varepsilon =0.05 at 450 oC, f= 89.9%. Some selective coatings with different layer thicknesses have been successfully optimized for different solar irradiations (air mass (AM0), AM1.5D, and AM1.5G spectra) and different operating temperatures (300, 450, and 600 oC), respectively. However, the optical constants for calculation are from the software, most datum are measured for bulk materials. Therefore, results are more useful to indicate the trend than the exact values.