Abstract: To study the deterioration law of mechanical properties of glass fiber reinforced composite (GFRP) bars in an alkaline solution, tensile tests were carried out using 20 mm and 25 mm GFRP bars for 30 days, 90 days and 180 days to obtain the mechanical properties such as ultimate tensile strength and elastic modulus after different corrosion time in the alkaline solution were obtained. Meanwhile, a comparative study on the mechanical properties of GFRP bars with the same diameters was conducted in saline solution for 180 days. The residual tensile strength degradation model was established for the GFRP bars in the alkaline solution. The results indicated that the deterioration degree of mechanical properties of GFRP bars in the alkaline solution was worse than that in the saline solution at the same corrosion age. The tensile strength of GFRP bars tended to decrease with an increase of soaking time in the alkaline solution, and their corrosion rate in the early stage was faster than that in the later stage.The modified residual strength degradation model of GFRP bars was in a good agreement with the test results.