Two-dimensional natural convection of a fluid of low Prandtl number (Pr = 0.02) in an annulus between two concentric horizontal cylinders is numerically investigated in a wide range of gap widths. For low Grashof numbers, a steady unicellular convection is obtained. Above a transition Grashof number that depends on the gap width, a steady bicellular flow occurs. With further increase of the Grashof number, steady or time-periodic multicellular convection occurs, and finally, complex unsteady convective flow appears. A plot is presented that predicts the type of flow patterns for various combinations of gap widths and Grashof numbers.