Microwave plasma-enhanced chemical vapor deposition (MW-PECVD) has been employed to synthesize carbon nanostructures by using Fe (or Co, Ni)/gamma -Al2O3 as catalysts and a mixture of benzene, hydrogen, and argon as precursors. By regulating the types of catalyst, the microwave incident power, the ratio and flux of the precursors, many morphologies such as ordinary geometric, helix-shaped, and planar spiral carbon nanotubes with aspect ratios of 100-1000 have been observed. Furthermore, two novel nanostructures, which are probably the missing link between onion-like carbon particles and nanotubes, have also been obtained. The striking feature of this new approach is the low synthesis temperature (< 520 degreesC) due to the non-equilibrium characteristic of microwave plasma operated at low pressure, which is crucial for some fascinating applications.