In the crystal growth process in solution, natural convec-tion occurs by a buoyancy force due to the density difference based on the temperature or the concentration difference. In this paper, the crystal growth of CdI2 in a saturated acetonic solution and aqueous solution of TGS (triglycine sulphate) was investigated. For the case where thermal and solutal convection occur in opposite directions, whether thermal or solutal convection is predominant is not only dependent on the thermal and solutal Rayleigh numbers, but is also affected by the Lewis number. Transient simulations of the double diffusive convection under normal gravity and microgravity were presented. Numerical results such as growth rate of the crystal and the velocity distribution under 1G agreed qualitatively with experimental results. The influence of convection clearly appeared in transient simulation under microgravity of 10−4G.