Growth of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films without initial nucleation was realized by an coaxial arc plasma gun at a substrate-temperature of 550 °C and hydrogen-atmosphere of 53.3 Pa. The pulsed arc discharge was triggered at a repetition rate of 1 Hz. The deposition rate was 80 nm/min. X-ray diffraction measurements with 12-keV X-rays from synchrotron radiation indicated extremely broad rings from diamond and none from graphite. The UNCD crystallite diameters were estimated to be approximately 1.3 nanometers by using Scherrer’s equation. The sp3/(sp2+sp3) was estimated to be approximately 57% from the X-ray photoemission spectroscopy. The coaxial arc plasma gun is a new powerful method that might enable us to realize the supersaturated condition with highly energetic ions for the growth of UNCD.