Water tank experiments were conducted to investigate plume rise and diffusion of gases discharged from stacks under calm conditions with field wind velocity less than 0.4m/s with stable thermal stratification. The plume rise phenomenon was illuminated LASER light sheet, and recorded video camera. The brightness of an image was transformed into concentration of a tracer liquid, by the equipment of digital image analysis. Plume rise height and plume lengths were measured from the concentration distribution of plume.