A small external magnetic field of 0.072T was reported to significantly influence the specific capacitances of the supercapacitor with magnetic carbon - metal (iron, cobalt or nickel) oxides microtubular nanocomposite fabrics as flexible electrodes. The decorated metal oxide species and annealing method (microwave-assisted vs. conventional tubular annealing) were discovered to play important roles in the increase or decrease of the capacitance. All the electrodes demonstrated excellent cycling retention performances, for example, upto ~350% for nickel oxide doped carbon nanocomposite electrodes. The corresponding mechanism for the magnetic field influence on the capacitance has been interpreted in term of a synergistic effect of three internal resistances, solution resistance in electrolyte solution, charge transfer resistance at electrode/electrolyte interface and leakage resistance in electric double layer region. The applied magnetic field is able to change the solution resistance due to the magnetohydrodynamic phenomena as well as the electrode resistance (directly corresponds to charge transfer resistance) by the magnetoresistance (MR) behavior.