The present work focuses experimentally on the study of free-convection heat transfer inside square tube with inner roughened surface at different uniform heat fluxes (q) and various inclination angles ( θ). ( θ) and (q) are performed at (0° ≤ θ ≤ 90°) and (51 W/m2 ≤ q ≤ 1198 W/m2). Rayleigh number (Ra) of the experimental tests is from 8.92 × 105 to 1.1 × 107. Results revealed that, at a certain Ra, mean Nusselt number (Num) increases with (0° ≤ θ ≤ 90°). Num of θ = 90° increases by 54% more than θ = 0° at 1198 W/m2. Roughened surface leads to decelerate the heat transfer by exposing the flow momentum to more drag-form and/or increase the flow blockage effect, but in contrast it augments more flow turbulence and helps to deform the boundary layer growth, thus produces thermal heat transfer enhancement. The roughened square tube achieves 5 –15% higher values of Nu m compared to smooth square-tube at θ = 0°. A comparative study between inclined smooth elliptic tube (Elshazly et al. Energy Convers. Manag. 46, 2005) and roughened square tube is also evaluated. Num correlations for vertical and inclined roughened square tubes are presented.