The lattice parameters of a highly stereoregular metallocene polypropylene crystallized at 145 degrees C were obtained after cooling and heating cycles in a temperature interval between 25 degrees C and 165 degrees C. The b dimension undergoes a large thermal expansion with temperature (0.6 Angstrom) while the change of the a axis is relatively small (0.1 Angstrom). The unit cell dimension along the molecular (c) axis appears less sensitive to temperature than are the intermolecular distances. The difference in dimensions between the a and c axis at low and high crystallization temperatures is small, varying from 2.3 to 3.5%. This small difference allows the formation of daughter, crosshatched lamellae in the complete interval of crystallization temperatures. The thermal expansion coefficient of the unit cell specific volume is also reported.