A model based on the cellular automaton (CA) technique for the simulation of solidification microstructure has been developed. An improved solid fraction calculating method is used, in which the solid fraction of the interface unit is calculated by temperature compensation method combined with local equilibrium phase diagram. And then, a quadratic equation of solid fraction can be calculated according to the local temperature, solute concentration and curvature. The method avoids the assumption of the position and shape of solid/liquid interface. By using this model on A356 aluminium alloy, a dendrite growth process is simulated. The model can predict the final microstructure both grain size and grain morphology. It also predicts the Si concentration distribution in both solid and liquid phases.