Boron doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition were structurally investigated. With an increase in the boron content, the grain size was increased from 5 to 23 nm accompanying by the lattice constant approaching to that of bulk diamond. The near-edge X-ray absorption fine-structure revealed that boron atoms are preferentially distributed into grain boundaries. On the basis of the results, the roles of the boron atoms in the enhanced crystalline growth are discussed. We consider that the crystalline growth posterior to the nucleation is facilitated by boron atoms existing neighbor to UNCD grains or by boron-containing energetic species in plasma.