The thermal behavior and the miscibility of an in-situ polypropylene blend named polypropylene catalloys (PP-cats) were investigated by using modulated differential scanning calorimeter (MDSC). It is found that all PP-cats samples present two glass transitions, one of which is ascribed to the ethylene-propylene random copolymer (EPR), and the other, to isotactic polypropylene (PP). However, no glass transition of ethylene-propylene block copolymer (E-b-P) responsible for a third component in PP-cats could be found. With the increase of EPR, the glass transition temperatures responding to PP and EPR components, T-g' PP and T-g' EPR, shift to low temperature, because of the enhancement of the interaction between PP and EPR component and the increase of ethylene content in EPR, respectively. Furthermore, the difference between T-g' PP and T-g' EPR remarkably decreases with the increase of the total ethylene content in PP-cats, which indicates that the miscibility of PP-cats is strongly dependent on the composition. Comparing the T-g' PP and T-g' EPR With T-g of fractionated PP and EPR, we ascribe the T-g, change of PP fraction to the increase of EPR content; while that of EPR, to the increase of ethylene content in EPR. These experimental results suggest that the existence of E-b-P plays an important role in improving the miscibility between propylene homopolymer and EPR in PP-cats. (C) 2007 Wiley Periodicals, Inc.