Abstract: Carbon microcoils (CMCs) with a fiber cross-section changing from flat to circular along with a coil diameter changing from 4.2 to 6.0 μm, which may develop a novel spring for micromechanical systems, were obtained by controlling the acetylene flow rate using Ni catalyst at a temperature of 1 013-1 053K. A growth model for these changes was proposed in which catalytic anisotropy is considered to be the possible factor changing the fiber cross-sectional shape of the CMCs. Owing to the change of catalytic anisotropy caused by reaction conditions, the same slender catalyst can produce both flat CMCs and circular CMCs depending on its relative orientation to the fiber axis. A cubic Ni catalyst can only produce a circular CMC, and can only change the coil diameter upon changing the reaction conditions. The model will give more insight into the formation of CMCs and provide information on the controlled synthesis of CMCs and carbon fibers.