The isothermal crystallization kinetics of poly(ϵ-caprolactone) (PCL) blocks confined in cylindrical microdomain structures (nanocylinders) formed by the microphase separation of PCL-block-polystyrene (PCL-b-PS) copolymers were examined as a function of nanocylinder diameter D and molecular weight of PCL blocks Mn. Small amounts of polystyrene oligomers (PSO) were gradually added to PCL blocks in PCL-b-PS to achieve small and continuous decreases in D. The time evolution of PCL crystallinity during isothermal crystallization at -42 °C showed a first-order kinetic process with no induction time for all the samples investigated, indicating that homogeneous nucleation controlled the crystallization process of confined PCL blocks. The half-time of crystallization t1/2 (inversely proportional to the crystallization rate) of PCL blocks with Mn ∼ 14 000 g/mol showed a 140-fold increase (from 0.48 to 69 min) by a 16% decrease in D (from 18.6 to 15.6 nm). Another set of PCL-b-PS/PSO blends involving slightly longer PCL blocks with Mn ∼ 15 800 g/mol showed a similar result. It was found by combining the results of two PCL-b-PS/PSO blends that the small increase in Mn (from 14 000 to 15 800 g/mol) yielded an approximately 90-fold increase in t1/2 (from 0.76 to 67 min) for PCL blocks confined in the nanocylinder with D = 18.2 nm. It is possible from these experimental results to understand the individual contributions of D and Mn to the crystallization rate of block chains confined in nanocylinders.