A finite-difference numerical study with a boundary-fitted coordinate system was made to investigate the three-dimensional, steady-state, laminar entrance flow of an axially uniformly heated horizontal tube with different adiabatic inserts. Parametric calculations have been performed to determine the nature and effect on thermally driven secondary flows of Reynolds number (100 less than or equal to Re less than or equal to 1000), Grashof number (10(3) less than or equal to Gr less than or equal to 10(6)), aspect ratio (1 less than or equal to AR less than or equal to 8) and radius ratio (0 less than or equal to R* less than or equal to 1). Typical streamline patterns, isotherms and axial velocity alone the flow direction are obtained. Local friction factor, Nusselt number and entrance length in the hydrodynamically and thermally developing, and fully developed regions are presented and discussed. Furthermore, how visualization was made and compared with the corresponding numerical results. The agreement is good for Re = 500 at the two different downstream distances.