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We have fabricated the multiwalled carbon nanotube (MWCNT) and nano-C doped MgB2 bulk and wire to investigate the effect of carbon substitution in the MgB2. Carbon substitution into boron site was confirmed by the shrinkage of the lattice parameter a with the increasing of the doping level. It was found that the lattice distortion and optimum doping level are different in the CNT and nano-C samples. This can be explained by carbon substitution, which is a more direct substitution process with the nano-C precursor. With CNT doping, carbon substitution only occurred after break out of carbon atoms from CNT. Both dopants showed significant enhancement in both magnetic critical current density, Jc(H), and transport critical current at high sintering temperature like 900degC . Transport Ic of magnitude 14 A and 30 A was observed at 4.2 K and 12 T for CNT and nano-C doping, respectively.