The material properties of fiber reinforced plastics (FRP) are strongly influenced by reinforcement fiber, so the strength degradation of the reinforcement is an important issue from a view point of long-term durability. For such purpose, present research focuses on the fiber strength degradation behavior of single fiber composite (SFC) specimen to quantify the strength degradation. The constant strain tests using the SFC specimen in water environment at various temperatures were conducted to reproduce the SCC of GFRP. Fragmentation test was conducted after the constant strain test, and the distribution of the residual strength of embedded fiber was obtained. It was clarified that the fiber strength degradation was to progress with higher strain, with time and was to accelerate dramatically at higher water temperature. Additionally, the degradation of the glass fiber strength has been quantified on the basis of microscopic SCC of the initial defects on the fiber surface using the sub-critical crack growth model.