Experimental investigations have been reported on steady-state natural convection from the outer surfaces of horizontal ducts with triangular cross sections in air. Two different horizontal positions are considered; in the first position, the vertex of the triangle faces up, while in the other position, the vertex faces down. Five equilateral triangular cross-section ducts have been used with cross-section side length of 0.044, 0.06, 0.08, 0.10, and 0.13 m. The ducts are heated using internal constant-heat-flux heating elements. The temperatures along the surface and peripheral directions of the duct wall are measured. Longitudinal (perimeter-averaged) heat transfer coefficients along the side of each duct are obtained for natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (perimeter-averaged) Nusselt numbers and the modified Rayleigh numbers are evaluated and correlated using different characteristic lengths. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers. Moreover, a dimensionless temperature group was developed and correlated with the modified Rayleigh number. For the upward-facing case, laminar and transition regimes are obtained and characterized. However, for the downward-facing vertex case, only the transition regime is observed. The local (perimeter-averaged) or the overall total Nusselt numbers increase as the modified Rayleigh numbers increase in the transition regime. However, Nusselt numbers decrease as the modified Rayleigh numbers increase in the laminar regime.