Helical carbon fibers were synthesized by the catalytic decomposition of acetylene as carbon source at low temperature of 240-260 degrees C with two nanocopper catalysts prepared by the hydrogen-arc plasma method and thermal decomposition of copper tartrate. Two growth modes for helical carbon fibers were obtained. One is mirror-symmetric growth mode, and the other is asymmetric growth mode. In the two growth modes, there were always only two helical fibers in regular shapes that were grown over a single copper nanoparticle. The two helical fibers had identical coil diameter, coil length, fiber diameter, cycle number, tight coil pitch, and cross section. In mirror-symmetric growth mode, the two helical fibers had absolutely opposite helical senses. The catalyst particle size was less than 50 nm and the coil diameter was <100 nm. Whereas in asymmetric growth mode, the two helical fibers had absolutely identical helical senses. The catalyst particle size was larger than 200 nm and their coil diameters reached 1 mu m. The catalyst particle size had considerable effect on the growth mode for helical carbon fibers.