Forced convective heat transfer in straight cylindrical ducts of parabolic cross-section under three most important thermal boundary conditions: (a) constant wall temperature (T boundary condition), (b) circumferentially constant wall temperature and axially constant wall heat flux (Hl boundary condition) and (c) both circumferentially and axially constant wall heat flux (H2 boundary condition) for a wide range of aspect ratios from 0.5 to 10.0 is reported in this paper. A finite difference method formulated in boundary-fitted non-orthogonal general curvilinear co-ordinates is used for the prediction of forced convective heat transfer in parabolic duct, the open face of which is covered by a flat plate. The hydrodynamically fully developed flow and the thermally developing flow region in this type of duct is considered for the prediction of friction factors and Nusselt numbers. The parabolic duct results are compared with those of the circular duct on the basis of constant hydraulic diameter and Reynolds number.