Pressure effect on the structure of proteins and peptides has recently been studied theoretically and experimentally. Some proteins are denatured under high pressure conditions while some peptides are more stable in the folded state than in the random coil state at high pressure. It is important to understand the molecular mechanism of pressure-induced structural changes of proteins and peptides.We studied pressure effect on the structure of peptides by using molecular dynamics simulations. Molecular simulations often get trapped in the local minimum states of the free energy. In order to overcome such difficulty, we used a generalized-ensemble algorithm, which gives more efficient sampling in a molecular simulation.We performed molecular dynamics simulations with a generalized-ensemble algorithm for helical peptides in explicit water molecules. We found that the population of the secondary structure of the peptides changes as pressure increases. We also calculated the free energy as a function of pressure. The detail molecular mechanism of the structural change induced by pressure will be shown in the meeting.