A camera based RGB three wavelength bands light extinction for fine particle size measurement is proposed. When a particle system is illuminated by a parallel white light, the transmitted light of different wavelengths can be analyzed by the light extinction theory to calculate the particle size. Using a color camera as a detector, the transmitted light is recorded in RGB three wavelength bands. The recorded signal is related to the camera′ s RGB response curve, light source spectrum characterization, particle size and concentration. Analyzing the RGB three wavelength bands signal with the light extinction theory, the particle size distribution can be determined. The simulations and experiments are carried out. And the results show the feasibility of this method. This approach combining image measurement with the light extinction method, enable the camera based particle size measurement ranged from sub micrometer to hundreds micrometers.