In this work, the durability of the E-glass/epoxy was investigated in terms of degradation in tensile properties and microstructure. Specimens were conditioned in seawater under sustained load and without load at 23 C, 45 C and 65 C for the period of 15 months. The tensile strength of the E-glass/epoxy reduced gradually with immersion time. It reduced by 11% from 811 MPa to 721 MPa and by 18.2% from 811 MPa to 663 MPa for samples immersed without load and with 15% sustained load respectively. The respective reduction for 23C and 45C was 1.1% and 5.5% for samples without load and 6.2% and 11.3% for samples immersion under 15% sustained load after 15 months of exposure. The failure strain of E-glass/epoxy composite was slightly affected for the samples immersed without load during the period of 15 months but it was more noticeable under 15% sustained load at 65C when the tensile strain reduced to 1.65% after 15 months of immersion. Furthermore, Scanning Electron Microscopy (SEM) analysis was conducted to evaluate the degradation of fiber matrix interface. Results revealed that moisture, temperature and sustained load have a deteriorative impact on the performance of the composite.