Abstract: 7781/CYCOM 7701 fiberglass/epoxy fabric prepreg was used to fabricate the composite lamina test specimens in medium temperature curing process. These test specimens were categorized into three groups, respectively corresponding to three types of environmental condition of testing:cold temperature dry(CTD), room temperature dry(RTD) and elevated temperature wet (ETW). Under the three types of test environmental condition, the composite lamina mechanical properties of tension, compression, shear, bearing and pull-through were tested. Meanwhile the mechanical properties difference between warp and fill direction of the composite, the existence of notch and impact damage, were properly considered in the test. Based on the test results, the effect of hygro-thermal condition on typical mechanical properties of 7781/CYCOM 7701 fiberglass/epoxy composite lamina was studied. Study indicates:Taking RTD condition as baseline, it rises in CTD condition but descends in ETW condition for each type of strength property. Of these, tensile strength descends 18% to 25%, compressive strength descends 10% to 40%, shear strength descends about 30% to 50%, bearing strength descends about 20%, and pull-through strength descends about 30% in ETW condition; Temperature and moisture absorption condition have a minor effect on tensile and compressive modulus, which is about 10% or below. But they have a major effect on Poisson's ratio and shear modulus. Taking RTD condition as baseline, it descends about 30% and 50% respectively for Poisson's ratio and shear modulus in ETW condition.