Abstract: A method for conveniently measuring the size distribution of hydrogen-bubbles in water electrolysis was presented based on micrograph, image proceeding and statistical analysis. Firstly, pictures of hydrogen-bubbles produced by water electrolysis in the same space and at different time were taken by an optical microscopy apparatus. Secondly, image enhancement, image segmentation, morphological transformation and particle measurements were adopted to get number and size micro-bubbles in each picture. Finally, based on ergodicity of the amount of micro-bubbles in measuring space, the size distribution of bubbles in the whole bubble-producing space was obtained by substituting statistical size distribution in time sequence for spatial size distribution. The experimental results show that the hydrogen-bubbles' size distribution in water electrolysis quite fits Gaussian function and the square correlative coefficients are all beyond 0.95 under three electrolytic currents, and the larger electrolytic current is, the more dispersive the size distribution is, but the influence of electrolytic current on average size of hydrogen-bubbles is little. Analysis and experiment make sure that the presented method can measure the size distribution effectively.