The effect of tacticity on the formation of the most ordered form of alpha 2 modification of crystals of isotactic polypropylene (iPP) during melt crystallization at atmospheric pressure has been investigated. It was found that melt crystallization of iPP with 99.5% isotacticity at 150 degrees C resulted in nearly 100% pure alpha 2 form crystals, i.e. fraction of alpha 2 (f(alpha 2)) = 1. At temperatures higher than 150 degrees C,f(alpha 2) was found to decrease from unity. The present study on the two different molecular weight samples showed that molecular weight does not influence the alpha 2 formation. The results are explained on the basis of the effect of chain mobility and thermal expansion on the regularity of interchain packing.