Two types of helical and straight carbon nanofibers, have been synthesized by the decomposition of acetylene using cupric carbonate basic as catalyst precursor at low temperature. The obtained carbon nanomaterials were characterized by transmission electron microscope, scanning electron microscope, and X-ray power diffraction. The size of the catalyst nanoparticles remaining inside the resultant nanofibers was determined. The carbon nanofiber diameters, ranging from 30 to 400 nm, closely correlated with the size of the catalytic nanoparticle. The growth mechanism of carbon nanomaterials was also studied. The nanocopper particle size had a considerable effect on the morphology of carbon nanofibers. The helical carbon nanofibers with a symmetric growth mode were synthesized with the nanocopper catalyst particles having a grain size less than 50 nm. When the average catalyst particle size determined was around 50–300 nm, the straight carbon nanofibers were obtained dominantly.