<b>Impact of Thickness of Diffusion Barrier on the Efficiency of Respiratory Organs in Relation to Body Weight in Freshwater Featherback, Notopterus Chitala Ham. </b> The present investigation is carried out to throw light on the oxygen uptake efficiency of water breathing organ, gills and air breathing organ, swim bladder which depends on the surface area and the thickness of the diffusion barrier of the respiratory membranes.The water blood diffusion barrier in the secondary lamellae composed of an outer layer of epithelium, a thin basement membrane and the innermost layer of flanges of pillar cell. The air blood diffusion barrier is composed of a single layer of epithelial cells and an underlying layer of blood capillaries. The water blood and air blood diffusion barrier were calculated to be 1.179 µm and 1.439 µm respectively in Notopterus chitala.In Notopterus chitala, the diffusing capacity of gills increased from 0.00094 and 0.07208 mlO2 min 1 mmHg 1 and of swim bladder from 0.00036 to 0.02446with gradual increase in body weight from 1.2 to 1435.0 g The slope value b were found to be 0.62113 and 0.64957 respectively for water breathing and air breathing organs.The weight specific diffusing capacity decreased from 0.78379 to 0.05023 and 0.30056 and 0.01705 mlO2 min 1 mmHg 1kg 1respectively for gills and swim bladder of Notopterus chitala for the same body weight range. The slope value b were calculated to be 0.37887 and 0.35043 respectively for water breathing and air breathing organ both.The estimated value for 1.0 g fish i.e, intercept a for respiratory organ were computed to be 1.02236 and 0.29452 respectively. Diffusing capacity, Body weight, Notopterus chitala 19-24 Issue-1 Volume-5 Ragini Kumari | Prabhat Kumar Roy | Tapan Kumar Ghosh