C-70 polymer films were prepared by rf plasma irradiation in Ar atmosphere. Laser desorption ionization time-of-flight mass spectra (LDITOF-MS) for the polymer films showed that C-2 loss occurred in the course of direct cross-linking of C-70 molecules. The polymer structure is discussed based on molecular orbital (MO) calculations of C-70 dimers at semiempirical levels, and the lower probability of direct cross-linking of C-70 molecules than that of C-60 molecules is suggested. The electric dark current of the plasma-polymerized C-70 was 10(-13)S/cm, compared to 10(-8)S/cm in the C-60 polymer prepared at the same plasma power. The almost-insulating electric conductivity of the C-70 polymer is attributable to the lower probability of conductive crosslinking of C-70 molecules originating from the electronic structure of C-70. The experimental and theoretical results suggest that the C-70 molecule is the smallest carbon nanotube with polyhedral caps and that the chemical properties of the caps closely resemble the C-60 molecule.