In this numerical investigation, a detailed comparison of the conjugate and non-conjugate natural convection within a semicylindrical cavity has been presented. The cavity is assumed to be ®lled with a ¯uid containing uniformly distributed internal heating sources. The bottom circular wall of the cavity is taken to be thick with ®nite conductive properties, while the top wall is considered to be isothermal. The Navier±Stokes and energy equations are solved numerically by using the SIMPLER algorithm. A Rayleigh number range from 3:2  10 6 to 3:2  10 11 has been investigated and the eects of solid-to-¯uid conductivity ratios of 1.0, 5.0 and 23.0 have been analysed. The present numerical results for a semi-circular cavity with entirely isothermal walls are compared with known results from the open literature. It was found that these results for the non-conjugate problem are in very good agreement. The present results for a conjugate cavity show a remarkable dierence from the non-conjugate analysis. The average Nusselt number for the solid±¯uid interface shows a decrease while the top wall average Nu number has increased. It has been concluded that these eects increase for a system with a low solid-to-¯uid conductivity ratio. It is evident from the present conjugate results that the assumption of isothermal enclosing walls gives somewhat dierent results when the walls are thick and the solid-to-¯uid conductivity ratio is small.