Aiming at on-line measurement of micro-bubbles’ size distribution in water, a proper image proceeding method was proposed. By regarding micro-bubbles’ pictures taken by micrography in laboratory as research object, the idiographic means such as image enhancement, image segmentation, morphological transformation and particle measurements adopted in the method were introduced, and the size distribution in a micro-bubbles’ picture was given. Experimental results and analysis show that the proposed method can obtain the size distribution of micro-bubbles in water effectually, and separate conglutination bubbles. The method is suitable for on-line measuring bubbles’ parameters in water.