Natural convection in a narrow horizontal concentric annulus is numerically investigated for a fluid of Pr = 0.4. The combined effect of thermal and hydrodynamic instability at Pr = 0.4 yields complex multicellular flow patterns and interesting transition phenomena. After the onset of instability of the conduction regime, four branches of solutions and five kinds of multiple solutions characterized by the temporal behaviour and spatial structure of flows are found. As Gr increases, the transition of flow structure shows a gradual increase in the number of cells. When Gr is further increased, the flow in the vertical section of the annulus reverts to a nearly monocellular structure. A transition phenomenon from oscillatory to steady flow is found with an increase in Gr.