The morphology changes in polyethylene (PE) and polypropylene (PP) melts were studied by rheometrical and x-ray methods. The dependence of viscosity on temperature exhibits reproducible, characteristic inflections within given temperature ranges. Throughout the transition range the variation of viscosity of PE is slower than in adjacent regions. The temperature dependences of the position, height, and half-width of the x-ray amorphous peak also show inflections in the same temperature range as viscosity. However, while the position of the amorphous peak and therefore the distance between chains varies in the transition range slower than in adjacent regions, the half-width and height asSociated to short range order and molecular chain mobility, respectively, change faster in the transition range than outside it. This means that morphological transformations at the molecular chains level, occurring in the transition range, have to be assigned to an increase of the length of parallel packed chain segments, as well as to the faster decrease of the short range order. The variation of melt viscosity and melt density supports this interpretation. Similar results were obtained for PP melts.