The effect of high hydrostatic pressure on collagen fibril formation was investigated. The turbidity curve for collagen fibrillogenesis, the surface hydrophobicity of collagen molecules, and the ultrastructure of collagen fibrils were studied. Although high-pressure treatment before fibrillogenesis had no effect on the rate of fibril growth, the treatment produced a denser network of collagen fibrils. High-pressure treatment after fibrillogenesis produced a looser network. Collagen fibrillogenesis was inhibited by pressurization to less than 200 MPa and accelerated by pressurization to more than 300 MPa. The surface hydrophobicity of the collagen molecules decreased with increasing pressure. Different pressures affected collagen fibril formation differently.