This paper deals with numerical prediction of natural convection heat transfer in the annular space between the circular receiver tube and the glass envelope in the so parabolic-cylindrical solar collector, for the case of an isothermal outer cylinder with a sinusoidal local heat flux distribution on the inner cylinder. The dimensional Darcy-Boussinesq's equations are formulated in bicylindrical coordinates and solved numerically using finite-difference based on successive-overrelaxation iteration. The parametric effect on the heat transfer characteristics is discussed with respect to eccentricity and azimuthal angular location of the inner cylinder for eccentric annulus. The two-dimensional structure of the fluid flow and temperature distributions as well as local and average Nusselt numbers arc obtained. It is demonstrated that the heat transfer in the annulus can indeed be optimized by a proper choice of the eccentricity.