This paper presents test results carried out to investigate the mechanical properties of E-glass fiber-reinforced polymer (E-GFRP) bars with four different diameters (6, 8, 1, and 14 mm) after these bars were subjected to sequential exposure to ultraviolet (UV) radiation and water vapor condensation for different time periods (1,, 2,, and 3, h) and elevated temperatures (15–3°C). The mechanical properties of the GFRP bars were evaluated with tensile, flexural, short beam (shear), and low-cyclic fatigue tests conducted on 15 specimens after exposure conditions. A scanning electron microscope (SEM) was used to investigate the degradation mechanism of the bars subjected to UV radiation and water vapor condensation. The results showed that, generally, with increases in exposure time and temperature the mechanical properties of the bars decreased. UV radiation and water vapor condensation conditions did not show a significant effect on direct tensile tests, in comparison with flexural and short beam tests. Furthermore, GFRP bars lost up to 45% of their tensile strength at elevated temperatures.