Experimental study of natural convection heat transfer inside smooth and rough surfaces of horizontal equilateral triangular channels with a uniformly heated surface are performed. The effect of smooth and rough surface of average roughness, ra = 0.02μm, on the heat transfer characteristics are studied. The local and average heat transfer coefficients and Nusselt number are obtained for smooth and rough channel at different Rayleigh numbers from 6.45 × 105 to 4.45 × 106. The findings show that the values of temperature difference between the inside surface and ambient air increase with increase of axial distance from both ends of the channel until a maximum value at the middle of the channel. The results show a higher values of local (Nux) for rough channel along the axial distance compared with the smooth channel. The average Num of rough channel is higher than Num of smooth channel by about 7%. The results obtained are correlated using dimensionless groups for both rough and smooth surfaces of the equilateral horizontal triangular channels