The main objective of the present work is to Study the properties of the fish by-products which result from the fish industry (Fillet tilapia) and the proportion of edible parts to non-edible parts tilapia fish from natural. Some physical properties of tilapia fish and its by-products .The drying behavior of Nile fish by- products was investigated in a mechanical dryer and the mathematical modeling by using thin layer drying model (Lewis's models) was used. The impact of the drying air temperature (60, 70 and 80 0C) ,hot air velocity ( 1.0 ,1.5 and 2 ms-1) and The thickness layer(5,7 and 9 mm ) on the fish moisture losses against drying time has been studied. Drying rate, moisture ratio, cost estimation and Chemical assays of the drying process were also considered. The obtained data showed that,average mass fish, skin, trimming, viscera, Head, backbone and meat 373.815, 12.002, 16.325, 34.631, 67.657, 54.88, 187.136 g respectively. The shortest drying time of the fish was recorded at 150 min with 80 0C drying air temperature, thickness layer 5mm and 2 ms-1 air velocity. While, the longest drying time was recorded at 510 min with 60 0C, thickness layer 9 mm and 1 ms-1 treatments .Moisture content decreased from an initial level of about 106.5% (d.b.) to range between 4.4and 2.2% (d.b.) at the end drying process depending on the drying conditions. Moreover, drying rate was increased with the increase of drying air temperature, air velocity and reducing the thickness of the layer. Between tested model, the modified simple exponential model was found to be the most suitable for describing the drying behavior of the fish by-products. The effects of drying air temperature and velocity on the drying constant and coefficient were also shown. On the other hands, the calculated operation cost of the dryer approaches about 2.92 LE/kg of dried whole fish by-product. The protein content in the final product 45%.