A micro/macro model is developed to represent the evolution of fluid flow, temperature field and microstructure formation in solidification processes. The macro model for fluid flow and heat transfer is established using the finite element method coupled with an Eulerian-Lagrangian formulation while the micro model for nucleation and grain growth is derived based on the principle of statistical physics. The macro and micro models are coupled through an iterative micro/macro lime step scheme. The numerical treatment is given. As an example, the integrated micro/macro model is applied to describe the evolution of fluid flow, temperature and the formation of solidification microstructure during the start-up phase in continuous casting of aluminum alloy. Calculated results compare reasonably well with measurements obtained ina commercial scale ingot for both the temperature distribution and the grain size distribution. The results indicate that the fluid flow and temperature fields:change drastically at the initial stage but evolve slowly afterwards, and also the average grain size increases with increasing distance from the ingot surface.