Abstract: By adopting renormalization group (RNG) kε turbulent model, a three dimensional numerical investigation, which considers the variable thermophysical properties and the coupling between the thermal conduction in the cavity wall and the natural convection of the air flow inside the cavity, was performed to study the characteristics of natural convection in fully opened cylindrical cavity. Three cases with different heating boundary conditions were examined: (I) a case with only bottom wall surface heated and side wall surface insulated, (II) a case with only side wall surface heated and bottom wall surface insulated, and (III) a case with bottom and side wall surfaces heated. The heated walls were maintained at constant heat flux. It was concluded that not only the wall heat flux and tilt angle of cavity are factors that strongly affect the temperature and velocity contours inside the cavity and in the opening surface, the mean temperature difference between bottom and side wall surfaces, the mean Nusselt number inside the cavity, etc., but also the wall heating boundary condition does. Furthermore, the natural convection for case I with only bottom wall heated has a great distinctive, in comparison with the case II and III both considered with side wall surfaces heated.