N-doped carbon nanotubes (CNTs) with ultra-large cavity have been synthesized by using a mixture of ZnO and graphite as catalyst in the floating catalyst method. The as-synthesized N-doped CNTs are very pure, and a striking characteristic structure is that the outer diameter is at least 10 times larger than the wall thickness. Moreover, electronic properties analysis reveals that the N-doped CNTs with ultra-large cavity have a reduced room temperature resistance compared with those of the common N-doped CNTs, which give an experimental prove for the previous theoretical predictions.