In a fiber/semicrystalline polymer system, interphasial transcrystallinities might form between the fibers and the matrix bulk that is composed of spherulites. By using an analogy method, the Young’s modulus E(T) of transcrystallinities in their growth direction was predicted by a well-known composite modeling theory. It is shown that E(T) is a function of the polymeric lamellar morphology and crystallinity. It is higher than the Young’s modulus E(s) of the bulk spherulites and positively affects the stiffness of the materials in which the transcrystallinities are formed. The study was focused on poly(ether ether ketone) (PEEK) but with polyethylene (PE) as a comparison. Also, it is demonstrated the various possibilities of the value E(T) and the ratio E(T)/E(S) for different polymeric matrices depending on their amellar moduli and the amorphous-phase ones.