Mixed convection heat transfer inside a vertical circular cylinder has been experimentally studied for upward and downward flows for hydrodynamically fully developed and thermally developing laminar air flow under constant wall heat flux boundary conditions for Reynolds number range from 400-1600 and the Grashof number range from 1.1 x 10(5)-7.4 x 10(6). The effects of the cylinder inclination angle and the flow direction on the mixed convection heat transfer process have been investigated. The experimental setup consists of an aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30). The hydrodynamically fully developed condition was achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths. The entrance sections were included two long calming sections, one with length of 1,800 mm (L/D = 60), another one with length of 2,400 mm (L/D = 80), and two short calming sections with lengths of 600 mm (L/D = 20) and 1,200 mm (L/D = 40). The results depict the surface temperature distribution along the cylinder axial length, the local and average Nusselt number distribution with the dimensionless axial distance Z for upward and downward flows. The results show that the surface temperature values for downward flow were higher than that for upward flow but it was lower than that for horizontal cylinder. The Nusselt number values were lower for downward flow than that for upward flow. It was found that an increase in Nusselt number values resulted in an increase in wall heat flux and as the angle of cylinder inclination moves from theta = 90 degrees (vertical cylinder) to theta = 0 degrees (horizontal cylinder). The mixed convection regime was bounded by Richardson numbers (Ri) which were approximately ranged from 0.1-30. The average heat transfer results were correlated with empirical correlations by dimensionless groups as Log (N) over bar(u) over bar against Log (R) over bar(a) over bar/(R) over bar(e) over bar for both upward and downward flows. The average heat transfer results were compared with the available literature and with forced convection flow.