Using a MgCl2-supported Ziegler-Natta catalyst, a series of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys with high EPR content (>20 wt %) were prepared by a periodic switching polymerization process (PSPP) in which the monomer feed was periodically switched between pure propylene and an ethylene/propylene mixture. The change of gas-phase monomer composition with time in the PSPP process was investigated. Transition periods were identified when switching the monomer feed from propylene to ethylene/propylene mixture (or vice versa). Because the gas-phase ethylene content in the transition periods was lower than that of the monomer mixture feed, copolymer formed in these periods has lower ethylene content than those formed in the steady periods of the copolymerization stage. As a result, the transition periods led to formation of more segmented ethylene/propylene copolymer (EPS) and decreased its ethylene content. When the switching frequency was altered in an adequate range, the content of EPS in the alloy was increased by increasing the switching frequency, and dispersion of the copolymer phase in the PP matrix became more uniform. The toughness-stiffness balance of PP/EPR alloy,synthesized by PSPP process with adequate switching frequency was better than the alloy synthesized by a conventional sequential polymerization process with only one copolymerization stage. When the switching frequency exceeded a certain limit, the alloy's EPS content was decreased and its toughness-stiffness balance tended to be worse.