Carbon nanospheres have been successfully fabricated by simple pyrolysis of a new class of carbon-based network polymers, poly(phenylcarbyne) and poly(hydridocarbyne). The analytic results of morphology and microstructure show that the resultant nanospheres are solid graphite-like balls with the diameter from several tens to hundreds of nanometers. A mechanism based on the carbon nanoparticles self-growth process is proposed for the formation of the nanospheres.