Potato flour extrudates were obtained by twin screw extruder. The effects of process variables were related to the following extrudate features: expansion, density, and shearing stress. The microstructure of the extruded products was examined by mercury porosimeter. Total porosity was related to physical properties of the extrudates. Total porosity decreased when expansion of products increased. Feed moisture was the principal determinante of physical strength and affected the changes of number, size and distribution of the pores.