A series of semicrystalline block copolymers, poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar microstructure have been synthesized. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, the crystallization of the PCL blocks in P4VP-PCL was carried out within the nanoscale confinement. Simply by varying the molecular weight of the block copolymer, namely the confined size, polymeric crystallization can be tailored in the one-dimensional confinement. A distinct nucleation mechanism, altering from heterogeneous to homogeneous nucleation, was obtained once the confined size became smaller than a critical dimension, equivalent to the regular thickness of heterogeneously nucleated crystalline lamellae. Consequently, discrete crystalline granules were generated through homogeneous nucleation, namely a single nucleus within one granule. Also, crystal growth was altered from specific to random orientation with respect to the interface between the crystalline and amorphous domains in the copolymers. This system thus serves as a model to analyze the impact of confined size in 1D spatial confinement on the crystallization of polymers.