Hygrothermal environment would cause severe degradation of property of unidirectional composite laminates. For the unidirectional composite laminates of ［90］₁₆ and ［0］₁₆ under different hygrothermal environment, the degradation of tensile properties was compared through tensile tests, and the mechanism of tensile property degradation was explained by fracture morphology and micromorphology analysis. The results show that, under dry condition, the epoxy matrix gets postcured at high temperatures. The tensile strength of ［90］₁₆ increases, while the tensile strength of ［0］₁₆ basically does not change with the increase of temperature; under 85%RH, thermal and moisture stress generate on the interface, the tensile strength of ［90］₁₆ and ［0］₁₆ unidirectional composite laminates decreases with the increase of temperatures; under 70 ℃ water bath, matrix resin and fibermatrix interface are serious damaged, and the tensile strength of ［90］₁₆ and ［0］₁₆ unidirectional composite laminates decreases significantly.