A particularly important defect related with solute segregation during solidification is the microporosity, which influences mechanical properties of castings. Considering only the formation of porosity due to shrinkage, during directional solidification, it is necessary a flow of liquid metal into the interdendritic channels to compensate for metal contraction, and pores are formed when the pressure drop in the liquid flow at a point within the mushy zone exceeds the pressure acting at this point. The increase in roughness of the interdendritic channels, caused by the successive ramification of the dendrite arms promotes a pressure drop inside the channel. In this paper a model developed by the authors is used to predict the permeability in directional solidification of Al-Cu and Sn-Pb alloys. From comparisons with experimental results models were choose to estimate primary and secondary dendrite arms spacing and applied to calculate the variation of permeability in directional solidification. From these results, applying a numerical method the pressure drop in the interdendritic channels was determined for the different alloys to analyze the influence of the composition of the alloy in the possibility of pore formation.