Laser beam welding (LBW) has become common practice in the production lines of several industrial sectors including the electronics, domestic appliances, and automotive industries. The advantages of LBW over conventional fusion welding processes (mainly GMAW and GTAW) is the lower welding heat input and smaller weld pool and HAZ dimensions, which are associated with lower residual stresses and distortion. In addition to the general problems encountered during the application of LBW on aluminumalloys (high reflectivity, porosity, loss of alloying elements), the most important problem, which concerns the heat treatable alloys, is the softening of the HAZ due to the dissolution and coarsening of the strengthening precipitates. The main objective of the present work is the simulation of the microstructural evolution in the HAZ in order to predict the hardness drop of the HAZ as a function of welding conditions. Models for the numerical simulation of precipitation, dissolution, and coarsening of β-Mg2Si phase were developed and solved with the use of the computational thermodynamics and kinetics software DICTRA. In this way the volume fraction and average precipitate size were calculated for several types of weld thermal cycles, under extremely nonisothermal conditions. Calculated hardness profiles in the HAZ are in good agreement with the experimental values. The above results point to the conclusion that it is possible to simulate the microstructure evolution and hardness in the HAZ of aluminum laser welds, thus opening the way for a more precise control and design of LBW of aluminum alloys.