A master thesis was discussed in Department of Mechanical Engineering / College of Engineering / University of Mosul entitled “Heat Transfer by Mixed Convection using Two Rotating Cylinders Inside an Insulated Enclosure Filled with Nanofluid” submitted by postgraduate student (Ahmed Ibraheem Farhan), Supervised By Assist. Prof. Ziad Mohammed majeed on Wednesday, Feb. 02, 2022.A numerical study of a mixed convection of heat transfer was studied for square enclosure containing two rotating cylinders filled with nanofluid. The nanofluid consists of basic fluid (water) and nanoparticles (Cu). The boundary conditions were adopted according to the geometric shape of the enclosure so that the right cylinder has a high constant temperature and the left cylinder has a low constant temperature while all enclosure walls are thermally insolated.The finite elements method was used to solving the governing non dimensional equations using Code (Comsol Multiphysics ® 5.6), It is one of the computational fluid dynamics programs (CFD).The effect of angular rotational speed (Ω) at (0, -3000, -6000 and -10,000), Richardson’s number at (10, 1 and 0.1), concentration of nanoparticles (F) at (0%, 0.2%, 0.4% and 0.06%), and diameter of the two cylinders (D1=D2) at (0.2, 0.3 and 0.4) have been studied.The results showed that the average Nusselt number increases with increases rotational speed, concentration of nanoparticles and diameter of the two cylinders by (75.17%), (3.37 %) and (72.4 %), respectively, while it increasing with decreases Richardson’s number by (13.37 %).The effect of changing the angular rotational velocity or changing the diameter of the two cylinders on the average Nusselt number is much greater than the effect of increasing concentration of nanoparticles in the base fluid.