This paper introduces a newly devised method of estimating the nuclei distribution in a cavitation tunnel. Applying this method, which requires no particular equipment, the nuclei distribution can be estimated by measuring the bubble cavity distribution on a test body in the test section of the tunnel. By the method the distribution of nuclei can be measured fairly accurately for larger nuclei which have major influence on the bubble cavity distribution, although the accuracy becomes poorer for smaller nuclei. The nuclei distribution was estimated by applying this method to photographes of bubble cavities on hemispherical and ITTC headforms tested in a cavitation tunnel at University of Tokyo. There was no direct measuring device at hand, so the estimated distributions at three different conditions were compared each other and it was found that this method was promising for rough estimation of nuclei.