The volume of lateral muscle, cross-sectional area of red and white fibers, and the number of fibers were examined for artificially hatched Bluefin tuna, Thunnus thynnus orientalis, from juvenile to young adult stage within the size range of 19.5-163.0 mm body length (BL). The red and white muscle volumes increased exponentially with BL. At a size larger than 80.0 mm BL, both the volume increases were significantly accelerated. The proportion of red muscle volume in the total lateral muscle volume slightly increased with BL. The cross-sectional area and the total number of red and white fibers at the point of maximum body height increased in the BL range examined. The small red fibers (<a 100 μm² in size) of a cross-sectional area gradually disappeared with the growth of BL. In contrast, there existed the white small fibers (200-300 μm² in size) at all body sizes. Increases of both fiber numbers were approximately accelerated at sizes larger than 85 mm BL. The size of 80-85 mm BL, at which the phase change of muscle development occurred, corresponded to the transitional stage from juvenile to young adult.