In current International Codes for FRP Reinforced Concrete, an environmental reduction factor is applied to the tensile strength of GFRP bar to account for its long-term durability. In this paper, the approaches for the durability design of GFRP bars are discussed and corresponding limitations are addressed, followed by presentation of a newly developed design approach, which incorporates the effects of relative humidity, exposure temperature, and design life. By using time extrapolation and time-temperature shift approaches, a new equation for design strength of GFRP bar under various exposure time and temperature was proposed. The effect of moisture, in the form of relative humidity, was incorporated into the new equation by investigating the relationship between the relative humidity and concrete pore water. On the basis of reported durability data for E-glass/VE GFRP bars embedded in moist concrete, reduction factors linked to service life, temperature and relative humidity were obtained. By utilizing the new approach presented in this paper, more refined and accurate design values for long-term tensile strength of a GFRP bar could be achieved.