Bimodal breathers have been of interest of many physiologists. Striped catfish (Pangasianodon hypophthalmus), known as facultative air breather, has high capacity for oxygen uptake from both water and air, thus it has been supposed to have well-developed gills and modified swim bladder. However, limit morphometric data are available for this. The aim of this study is to evaluate gill morphology and morphometric of P. hypophthalmus at different sizes. Particularly, striped catfish at juvenile size (15-20g; n=10), small size (50-100g; n=10), medium size (300- 400g; n=10) and large size (800-1000g; n=6) will be selected and stocked in the same condition at 27^oC and 92% of oxygen saturation for 30 days. After that, fish weight and length will be measured. In addition, total volume of gill filaments, total volume, and respiratory surface area of secondary lamellae of these fish will be investigated by stereology – based methods. Results regarding these metrics at different sizes of fish as well as correlations among these metrics will be presented and discussed. The results from this study would give a new insight about the most iconic species in Mekong delta areas in Vietnam by outlining an overview of morphological and physiological characteristics of the main respiratory organ of this species, in addition to setting a baseline for further studies.