Hydrogen absorption in metallic nanoparticles was investigated by classical molecular dynamics (MD) simulation. We used a simple model composed of an isolated f.c.c. or b.c.c. nanoparticle of 1, 1.4, 2, 4, 6, 8 and 10 nm in diameter and surrounding hydrogen atoms. The simulated particle sizes are which correspond to about 50 to 44000 atoms. In the case of f.c.c. nanoparticles, atomic configuration with five-fold symmetries was observed in both hydrogenfree and hydrogenated particles smaller than 2 nm. The f.c.c. structure was maintained in larger particles than 4 nm with lattice deformation which varies with M-H interaction. The b.c.t. structure was observed in hydrogenated b.c.c. nanoparticles. Number of H atoms absorbed in a nanoparticle varies depending on particle size and M-H interaction: it increases with increasing particle size and M-H bond strength.