Marangoni-Benard convection which is mainly driven by the thermocapillary (Marangoni) effect occurs in a thin liquid layer heated uniformly from the bottom. The wavenumber of supercritical convection is studied experimentally in a 160 x 160-mm(2) cavity that is heated from the bottom block. The convection pattern is visualized by an infrared thermography camera. It is shown that the onset of the Benard cell is consistent with theoretical analysis. The wavenumber decreases obviously with increasing temperature except for a slight increase near the onset. The wavenumber gradually approaches the minimum when the supercritical number.. is larger than 10. Finally a formula is devised to describe the wavenumber selection in supercritical convection.