The aim of this study was to characterize the nanostructures formed from assembly of poly(ethylene oxide)-bpoly( α-benzyl carboxylate ε-caprolactone) (PEO-b-PBCL) in water, determine the effect of weight fraction of the hydrophilic block( fEO) on their morphology, and to investigate their potential for solubilization and delivery of P-glycoprotein (P-gp) inhibitor, valspodar. Three PEO-b-PBCL block copolymers having fEO ranging from 0.18-0.40 were synthesized. Assembly of PEO-b-PBCL was triggered through a co-solvent evaporation method. The average critical aggregation concentration (CAC) for PEO114-b-PBCL₃₀, PEO₁₁₄-b-PBCL₆₀, and PEO₁₁₄-b-PBCL₉₅ was found to be 62, 41, and 23 nM, respectively. A lower rigidity of the hydrophobic domain in nanostructures formed from the assembly of PEO₁₁₄-b- PBCL₆₀ and PEO₁₁₄-b-PBCL₉₅ in comparison to PEO₁₁₄-b-PBCL₃₀ was observed. The morphology of the assembled structures was characterized by transmission electron microscopy (TEM). The TEM images of PEO₁₁₄-b-PBCL₃₀ (fEO = 0.40) showed the formation of spherical micelles with high polydispersity, whereas the assembly of PEO₁₁₄-b-PBCL₆₀ (fEO = 0.25) and PEO₁₁₄-b-PBCL95 (fEO = 0.18) resulted in a mixed population of spherical micelles and vesicles. Valspodar achieved high loading in all the three PEO-b-PBCL nanocarriers reaching aqueous solubility of nearly 2 mg/mL. The morphology of PEO-b-PBCL carrier did not seem to influence the pharmacokinetics of the encapsulated valspodar in rats following intravenous administration. In conclusion, the results show a potential for PEO-b-PBCL nanocarriers as efficient solubilizing agents for delivery of valspodar.