Natural convection of air with Pr=0.7 between two horizontal plates with small magnitude non-uniform temperature distribution[${in}{Delta}Tsin({kappa}X/H)$, H : gap width, X : horizontal coordinate] in the lower plate is numerically(${in}=0.01$) investigated. In the conduction-dominated regime with $Ra{leq}1700$, two upright cells are formed over one wave length($2{pi}/{kappa}$). 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 ${kappa}=1$, a transition of $2{ ightarrow}4{ ightarrow}6$ eddy flow occurs with increase of Rayleigh number, and no hysteresis phenomenon is observed. Dual and triple solutions are found for ${kappa}=1$, and transitions of $10{ ightarrow}8$, $8{ ightarrow}6$, $6{ ightarrow}4{ ightarrow}2$ eddy flow occur with decrease of Rayleigh number.