Natural convection of air with Pr=0.7 between two horizontal plates with small magnitude non-uniform temperature distribution[?ΔT sin(kX/H), H : gap width, X : horizontal coordinate] in the lower plate is numerically(?=0.01) investigated. In the conduction-dominated regime with Ra ≤ 1700, two upright cells are formed over one wave length(2π/k). For small wave number, the flow becomes unstable with increase of Rayleigh number, and multicellular convection occurs above a critical Rayleigh number. The flow patterns are classified by the number of eddies over one wave length. When k=1, a transition of 2→4→6 eddy flow occurs with increase of Rayleigh number, and no hysteresis phenomenon is observed. Dual and triple solutions are found for k=1, and transitions of 10→8, 8→6, 6→4→2 eddy flow occur with decrease of Rayleigh number.