The molecular weight distribution (MWD) of polymer blocks constituting block copolymers (BCPs) directly influences their self-assembly in the bulk and the resulting periodic nanostructures. However, the effect of the MWD of core-forming hydrophobic blocks on the structures of BCPs self-assembled in solution has rarely been studied in relation to the formation of highly ordered mesophases. Herein, we report the effects of core-forming hydrophobic block MWD on the formation of bicontinuous cubic mesophases of BCPs in solution. Specifically, a series of BCPs with tunable-MWD hydrophobic blocks is prepared by coupling a discrete (Đ = 1.0) branched poly(ethylene glycol) (bPEG) hydrophilic block with a hydrophobic polystyrene (PS) block with the desired MWD. The MWDs of PS blocks were adjusted by controlling the addition rate of the initiator for the atom-transfer radical polymerization of styrene, with the resulting Đ values ranging from 1.08 to 1.72. The solution self-assembly of the thus produced branched-linear BCPs (bPEG-PS) indicated that an increase of PS block Đ value from 1.09 to 1.72 resulted in a gradual deterioration of the internal order of bPEG-PS polymer cubosomes from a cubic crystalline phase to a disordered cubic and sponge phase. It was concluded that the MWD of the core-forming hydrophobic block is the major parameter determining the internal structure disorder of polymer cubosomes. In addition, when the Đ value of the core-forming hydrophobic PS block of bPEG-PS was increased at constant Mn, the dominant symmetry of self-assembled cubosomes changed from Pn[3 with combining macron]m to Im[3 with combining macron]m.