The transition metal carbides and nitrides (called MXenes), have garnered great attention due to their versatile surface chemistry and unique layered structure. The terminated functional groups of MXene are critical in developing intriguing properties to the increasing demand of intelligent control systems. This study demonstrates the synthesis of oxidized Nb2CTx MXene via two procedures, a hydrothermal process (h-Nb2O5/C) or CO2 calcination (c-Nb2O5/C). The obtained Nb2O5/C exhibits a hierarchical architecture with Nb2O5 nanoparticles decorated on the surface of disordered carbon layers. Attractively, the electrorheological (ER) performance of h-Nb2O5/C based ER fluid is superior to that of the c-Nb2O5/C based one. Combining the XPS, EDX and BET results, we speculate that the outstanding ER behavior is related to the following synergistic effects: large oil-accessible surface of the disordered carbon as well as high dielectric constant of the Nb2O5 nanoparticles. Notably, the h-Nb2O5/C based ER fluids possess a stable plateau region of shear stress throughout a wide range of shear rate, which is valuable for a host material of practical engineering applications.