A series of diblock copolymers (PS-b-PBBHCSs) composed of polystyrene (PS) and poly(2,5-bis{[6-(4-butoxy-4'-oxybiphenyl)hexyl]oxycarbonyl}styrene) (PBBHCS) were successfully synthesized via atom transfer radical polymerization, with their chemical structures confirmed by various characterization techniques. While PS is a conventional coil polymer, PBBHCS is a novel combined main-chain/side-chain liquid crystalline polymer (MCSCLCP) which can exhibit hierarchically ordered structures. Small-angle X-ray scattering results show that the diblock copolymers with different compositions could present lamellar and rectangular cylinder morphologies before the order-disorder transition. Combining the differential scanning calorimetry and X-ray scattering results, we find that the PBBHCS block in the confined space of microphase separation is still able to exhibit its liquid crystalline (LC) structures with the rectangular main-chain scaffold on the nanometer scale and the side-chain ordering on the subnanometer scale, which is the same as that of the homo-PBBHCS. Therefore, a three-length scale ordered hierarchical structure can be observed in the system we studied, namely, the microphase separation and the double-length scale LC structure of the MCSCLCP. The LC structure of PBBHCS is more stable in the diblock copolymer than that in the homopolymer with similar molecular weights. Therefore, not only PS-b-PBBHCS can fabricate more complex hierarchy structure but also the microphase separation can stabilize the hierarchical assembly of PBBHCS in the nanodomains.