An YNi 2 B 2 C thin film grown on a MgO(001) substrate by pulsed laser deposition has been investigated by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Planview TEM analyses show that the film consists of isolated rectangular YNi 2 B 2 C grains distributed within a second phase. This phase was identified as monoclinic Y 2 Ni 15 B 6 with lattice parameters a ) 1.42 nm, b ) 1.07 nm, c ) 0.96 nm, and ) 95°. Within the YNi 2 B 2 C grains, a hexagonal Y 0.915 Ni 4.12 B phase with lattice parameters a ) 1.49 nm and c ) 0.69 nm is intergrown with a cubic Y 2 O 3 phase with lattice parameter a ) 1.06 nm. The orientation relationships between YNi 2 B 2 C, Y 2 O 3 , and Y 0.915 Ni 4.12 B are analyzed, and the mechanisms of phase formation are discussed.