The performance of castings is primarily dependent on the solidification microstructures and defects. Gas porosity is one of the major casting defects existing in the castings of aluminium and magnesium alloys. In this work, a two－dimensional (2D) cellular automaton (CA) model is proposed to simulate dendrite and microporosity formation during solidification of alloys. The model involves three phases of liquid, gas and solid. The effect of liquid－solid phase transformation on the nucleation and growth of porosity, the redistribution and diffusion of solute and hydrogen, and the effects of surface tension and environmental pressure are taken into account. The growth of both dendrite and porosity is simulated using a CA approach. The diffusion of solute and hydrogen is calculated using the finite difference method (FDM). The simulations can reveal the coupling and competitive growth of dendrites and microporosities, as well as the microsegregation of solute and hydrogen. The model is applied to simulate the microporosity formation during solidification of an Al－7%Si (mass fraction) alloy. The effects of initial hydrogen concentration and cooling rate on microporosity formation are investigated. The results show that the simulated pressure difference between the inside and outside of a porosity as a function of the reciprocal of porosity radius obeys the Laplace law. With the increase of initial hydrogen concentration, porosity volume fraction increases, and the incubation time of microporosity nucleation and growth decreases, while the porosity density does not increase obviously. With cooling rate decreasing, porosity volume fraction and maximum porosity radius increase, as well as porosity nucleates and starts to grow at higher temperatures. However, the porosity density shows a decreasing trend with the decrease of cooling rate. The competitive growth between different microporosity and dendrites is observed. The porosity nuclei with larger size are able to grow preferentially, while the growth of the small porosity nuclei is inhibited. Because of the effect of gas－liquid surface tension, porosity grows spherically when it is enveloped by liquid. After touching with dendrites, the growth space of porosity is restricted by the complex dendrite network, and thus becomes irregular shape. On the other hand, the growth of dendrite might also be influenced by the nearby porosity. With cooling rate decreasing, the competitive growth between porosities and dendrites becomes more evident, leading to non－uniform porosity size, and more irregular morphology of the porosities with larger size. The simulation results are compared reasonably well with the experimental data.