A spherical TiCl4/MgCl2-based catalyst was used in the synthesis of polyethylene/polypropylene/poly (ethylene-co-propylene) in-reactor alloys by sequential homopolymerization of ethylene, homopolymerization of propylene, and copolymerization of ethylene and propylene in gas-phase. Different conditions in the third stage, such as the pressure of ethylene-propylene mixture and the feed ratio of ethylene, were investigated, and their influences on the compositions, structural distribution and properties of the in-reactor alloys were studied. Increasing the feed ratio of ethylene is favorable for forming random ethylene-propylene copolymer and segmented ethylene-propylene copolymer, however, slightly influences the formation of ethylene-b-propylene block copolymer and homo-polyethylene. Raising the pressure of ethylene-propylene mixture results in the increment of segmented ethylene-propylene copolymer, ethylene-b-propylene block copolymer, and PE fractions, but exerts a slight influence on both the random copolymer and PP fractions. The impact strength of PE/PP/EPR in-reactor alloys can be markedly improved by increasing the feed ratio of ethylene in the ethylene-propylene mixture or increasing the pressure of ethylene-propylene mixture. However, the flexural modulus decreases as the feed ratio of ethylene in the ethylene-propylene mixture or the pressure of ethylene-propylene mixture increases. (c) 2006 Wiley Periodicals, Inc.