Heat transfer study has been carried out during natural convection boiling in an internally heated vertical annulus. The test section consists of a stainless steel tube, enclosed in a corning glass tube, forming an annular space through which the working fluid (water) flows. The S.S. tube is electrically heated. The experimental measurements have been made of wall temperatures along the length of the stainless steel tube and the inlet-outlet temperatures of the working fluid at the test-section, separator and condenser. Temperatures and mass flow rates of the cooling water entering and leaving the condenser, have also been recorded. The experiment was carried out for different heat fluxes impressed over the steel tube and repeated for two submergence levels of the liquid in the down flow pipe. Local and average heat transfer coefficients of the water film on the stainless steel tube surface, have been determined. Subsequently, the non dimensional parameters governing heat transfer and buoyancy induced flow processes, have been calculated and exhibited graphically. Correlations between the various parameters have also been developed.