A numerical study on the natural convection of small Prandtl number fluid in a cylindrical enclosure with lateral heating has been performed by a combination of second order time-splitting and pseudo –spectral methods. The basic state and its stability is analyzed in a horizontal cylindrical cavity of finite length H and radius R H/2R 2. The effects Rayleigh number and aspect ratio (length/diameter) on the nature and origin of instabilities are investigated.