A series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers were designed and synthesized to study the change in crystal orientation of PEO blocks under different confinement sizes. The volume fraction of PEO blocks (f(PEO)) in these copolymers was kept almost identical (the fPEO values were between 0.45 and 0.48) but with different number-average molecular weights for the PS and PEO blocks ((M) over bar (PEO)(n) and (M) over bar (PS)(n)). Therefore, the phase morphology of these copolymers was a lamellar structure with different PEO and PS layer thicknesses (d(PEO) and d(PS)) detected by synchrotron small-angle X-ray scattering (SAXS) experiments. Since the melting temperature of these PEO crystals is lower than the glass transition temperature of the PS layers, the PEO block crystals could be melted and recrystallized under one-dimensional (1D) confinement at different crystallization temperatures (T,). The PEO block crystal orientation changes were monitored using synchrotron wide-angle X-ray diffraction (WAXD) experiments. It was found that the crystalline PEO chain orientation (the c-axis in the crystals) underwent a change from being perpendicular (homogeneous) to the layer normal direction (h) at low T(c)s to parallel (homeotropic) at high T(c)s in these 1D confined samples with the d(PEO) ranging from 8.8 to 23.3 nm. However, with the gradual release of this 1D confinement, i.e., increasing the dPEO, a broad T-c legion in which the inclined c-axis orientation was originally observed in the PEO-b-PS with the low (M) over bar (PEO)(n) (8.7K g/mol) and (M) over bar (PS)(n) (9.2K g/mol) became increasingly narrowed by pushing the starting T, where the tilting initiates toward higher T-c and reducing the ending T-c where the parallel orientation of the c-axis with (n) over cap starts. In the PEO-b-PS sample with the highest M-n(PEO) (57K g/mol) and (M) over bar (PS)(n) (61.3K g/mol) in this study, this T. region was narrowed to less than 5 degreesC, suggesting the confinement size effect on the crystal orientation of the PEO crystals. The homogeneous to homeotropic orientation change of the c-axis in the PEO crystals with increasing Tc was explained to be largely governed by the primary nucleation and crystal growth processes of the PEO blocks for developing the maximum crystallinity. A semi-quantitative calculation was attempted to illustrate why the homogeneous orientation of the PEO crystals takes place in the 1D confinement based on the SAXS, WAXD, and differential scanning calorimetric results. It was expected that when the d(PEO) becomes large enough, the homogeneous orientation of the c-axis in the PEO crystals would disappear.