A mesoscale dynamic thermodynamic sea ice model is presented. The model differs from the large scale sea ice model in both the dynamic and thermodynamic parts. In the dynamic calculation, a constitutive relation based on ice floe collision is used for the determination of the internal ice force in the Marginal Ice Zone (MIZ), while in the thermodynamic calculation, a three-level approach for the ice thickness distribution is adopted instead of the common two-level approach . The simulation results from the mesoscale sea ice model show that the three-level approach is more sensitive to geophysical forcing than the two-level approach. Thus it is demonstrated that the mesoscale sea ice model has the potential of a more accurate description of the dynamics and the thermodynamics than the previous models in the simulation of sea ice processes in subpolar regions. Giving the exact initial conditions and the atmospheric and oceanic forcings, this model can be used as a sea ice forecasting model in subpolar regions. ¿ American Geophysical Union 1990