Abstract: The morphological changes of mitochondria rich cells (MRC_S) in gills of juvenile goldfish (Scatophagus argus) were studied by microscopic technique under the stress of sudden salinity drop (20 salinity group put into 5 salinity group) and salinity sudden rise (20 salinity group put into 35 salinity group). The results show that the long and short diameters of MRCs were (9.517±1.390) μm and (7.150±1.448) μm, respectively, significantly higher than those of the control which were (7.317±0.986) μm and (7.317±0.986) μm (P<0.05), respectively; and the MRCs number increased significantly at 6^th hour. Two types of MRCs were discovered: Type I MRCs, oval in shape with irregular nuclei, well-developed inner ridges and closed or open apical openings; Type II MRCs, round, with regular round nuclei and multiple apical openings. With different shapes of mitochondria, Type a mitochondria were slightly stained, showing short and thick shape; Type b mitochondria were more deeply stained, showing small granular. The results indicate that juvenile S. argus will produce a response mechanism through the changes of MRCs volume, quantity, shape and structure, as well as the shape and structure of mitochondria in order to maintain the stability of internal environment.