An admixture of isotactic poly(1-butene) (iPB) with isotactic polypropylene (iPP) enhances the scope of consumer applications and the commercialization of iPB because of the reduced price. However, the properties of the mixtures are not superior to but are intermediate between those of the components. Achieving high-performance iPB materials by blending without sacrificial advantages of each component is still a great challenge. In this work, the iPB/iPP in-reactor alloys, as novel polyolefin materials, are differentiated from the traditional mixtures by the existence of the in-situ-synthesized poly(propylene-co-butene). The copolymer enables improved compatibility between iPB and iPP and increased interfacial entanglements between their spherulites during crystallization. Compared with iPB/iPP blends, all of the iPB/iPP alloys exhibit higher overall performance, even exceeding the respective characteristics of iPB and iPP materials.