We choose nano-Pt in hydrogen environment to explore the size effect on the formation of metal hydrides.At 30 GPa,a phase transition in the metal lattice from the cubic to hexagonal phase is observed characterized by a drastically increased volume per metal atom,indicating the formation of PtH-P63/mmc.We find that nano-Pt could form PtH at a lower pressure than the bulk Pt due to its high specific surface and structure defects.The present work provides the possible route to new metal hydrides under mild conditions.