The small scale structure of free convection at high Rayleigh number was investigated by a flow visualization technique and temperature measurements. The dominant feature of the flow field was many sheetlike plumes consisting of several thermals, which transport heat from the conduction layer to the interior region. It was found that the boundary heat flux and the fluid’s properties determine the average characteristics of the thermals, namely, (a) the distance from the boundary where the thermals are generated, (b) the temperature excess transported by thermals, (c) the upward velocity of the fluid in a thermal, and (d) the horizontal dimension of the thermal. Velocity fluctuations and the heat transport rate can be explained through these characteristic values.