Numerical simulation study by using Fluent CFD software has been carried out to investigate the steady laminar natural convective heat transfer for air formed by heated inner equilateral triangular cylinder inside cold circular enclosure. The triangular cylinder was located at center of enclosure with four radius ratio RR=1.5, 2, 2.5, and 3; and four inclination angles, 𝜃=0o (the base of triangle cylinder at bottom), 20o , 40o and 60o (the base of triangle at top). All numerical calculations were performed in the range of Rayleigh number extends from 10 3 to 10 7 . The fluid and temperature fields were represented in the form of streamlines and isotherms. Results show that, As Rayleigh number increases, the streamlines become more concentrated next to the walls and the center of vortex displays upward towards the central plane of enclosure. Also, there is no effect for the inclination angle of triangular cylinder on the peak values of local Nusselt number along the inner surface of circular cylinder except the positions of these peaks due to turning of thermal plumes.