Ultracapacitors of asymmetric configuration have been prepared with activated carbon (AC) and undoped or Mo-doped manganese oxide (MnO2) in 1.0M Na2SO4 electrolyte. Phase analysis shows the AC powder, 1–15μm in size, contains both disordered and graphitic structures, and the undoped and Mo-doped oxide powder, 0.05–0.20μm in particle size, mainly involves amorphous MnO2 and MoO2. CV results indicate the single electrode of AC plus 10wt% Mo-doped MnO2 (A9OM1) is superior to the electrode with undoped MnO2 or high content of doped MnO2, exhibiting features of double layer capacitance at high scan rate and pseudocapacitance characteristics at low scan rate. When assembled with a negative electrode of AC, the capacitor of positive A9OM1 electrode demonstrates the least power loss among three asymmetric capacitors. This asymmetric capacitor also shows a higher capacitance than the symmetric AC capacitor when the current density is less than 8.0Ag−1 in 1.8V potential window. But a higher electrode resistance of A9OM1, in contrast with AC, compromises its capacitance plus. When the energy density of A9OM1 asymmetric capacitor is compared with that of symmetric AC capacitor at the same power level, the capacitance benefit on energy density is restricted to current density≤3.0Ag−1.