An experimental investigation has been conducted to study the steady-state forced-convection heat-transfer characteristics of the hydrodynamic fully-developed turbulent flow in air-cooled horizontal equilateral triangular ducts (i.e. of 60° apex angle), which were each fabricated with the same length of 2.4 m and hydraulic diameter of 0.44 m. The inner surfaces of the triangular ducts were roughened by a milling process, shaping process or fixing uniformly-spaced parallel square ribs orthogonal to the mean air flow. The average surface roughness of the inner surfaces, which were produced by milling and shaping processes, were 3.0 and 11.5 μm, respectively. The square-sectioned ribs, adopted to produce the roughened surface, had different protrusions of 6.35, 9.525 and 12.7 mm, and the uniform separation between the centre lines of two successive ribs was kept constant at 57.15 mm. Both the triangular ducts and the square ribs were fabricated out of duralumin. The experiments were performed with the hydraulic-diameter based Reynolds numbers ranging from 4000 to 15000. The entire inner wall of the duct was heated uniformly, while its outer surfaces were thermally well insulated. By comparing the heat-transfer performances with those of a smooth triangular duct (i.e. average inner-surface roughness of less than 1.0 μm) having the same geometry, it was found that forced convection was enhanced by the roughened surfaces. In addition, a much enhanced forced convection was obtained by fixing uniformly-spaced parallel square ribs, rather than by fabricating random roughness on its inner surfaces by machining. However, the heat-transfer enhancement was not proportional to the rib size; the maximum forced convection heat-transfer augmentation was obtained using the smallest (i.e. 6.35 mm) ribs of those tested. Non-dimensional expressions for the determination of the steady-state heat-transfer coefficient of the equilateral triangular ducts, which were fabricated with the various kinds of artificial inner-surface roughness, were also developed.