An unsteady numerical model based on thermosolutal buoyancies with Soret and Dufour effects for double-diffusive convection is developed. The thermosolutal model is discretized by the finite volume method and solved numerically using the SIMPLE algorithm with QUICK scheme in non-uniform staggered mesh. The flow field, temperature and concentration distributions for different aspect ratios, buoyancy ratios, Rayleigh numbers, Soret and Dufour coefficients are investigated systematically. The results show that the flow structure of different aspect ratios develops from conduction-dominated to steady convection-dominated, and finally evolves into periodic oscillatory convection as buoyancy ratio or Rayleigh number increases. The vortex number of flow structure, recirculation zones of isotherm and isoconcentration contours reduce along the transition route while both of them increase as aspect ratio decreases. The average Nusselt and Sherwood numbers keep constants during the conduction-dominated stage, but increase with increasing Rayleigh number, increasing buoyancy ratio or decreasing aspect ratio during the convection-dominated stage. (C) 2014 Elsevier Ltd. All rights reserved.