A simplified analytical solution, numerical calculations with the use of the FLUENT/UNS code and experimental studies of laminar free-convection heat transfer from an isothermal hemispherical cavity in unlimited space are presented. The analytical solution is based on an adaptation of the methods used for inclined isothermal plates. In the proposed solution, the control surface of the hemisphere was considered as a small inclined surface. Inclination of this surface was a function of azimuth angle. The results of theoretical analysis, numerical calculation and experimental procedure are presented in the Nusselt and Rayleigh number: relations Nu(D)=0.296 Ra-D(1/4), Nu(D)=0.340 Ra-D(1/4) and Nu(D)=0.316 Ra-D(1/4) respectively. The comparison of theoretical and numerical solutions with experimental results presented in this paper shows, good agreement.