The geometry optimizations and evaluation of electronic properties of prism carbon series were performed based on the density functional theory using plane waves with the PBE functional. The most stable structure was a prism-C-5 tube. Since the band gaps of these carbon tubes, as estimated by band structure calculations ranged from zero to 0.96 eV, it is considered that the electronic characteristics change from semi-conducive to metallic with increasing number of carbon atoms per unit cell. Since the activation energies of prism-C-4 and -C-6 tubes are larger than the thermal energy, estimated from k(B)T, these structures can exist at ambient temperatures.