The effect of alkaline environmental condition on durability of GFRP composites according to additives was investigated. Additives used were polyvinyl alcohol(PVA), kaolin and alumina powder. Weight gains increased with immersion time in all GFRP composites at $80^{circ}C$. But weight gain of specimen added PVA did not differ through the wlwle immersion time in both tap water and alkaline solution at 20 and $80^{circ}C$. Tensile strength decreased with immersion time in all environment conditions. Tensile strength of GFRP composites regardless of additives decreased rapidly up to 5 days of immersion and then decreased slowly up to 30 days in alkaline solution environment at $80^{circ}C$. Weight gains had not. much difference in both tap water and alkaline solution at $20^{circ}C$. And weight gain of GFRP composites added polyvinyl alcohol had smaller than the others through the whole immersion time in both tap water and alkaline solution at $20^{circ}C$ and $80^{circ}C$. Tensile strength of GFRP composites added polyvinyl alcohol had higher than the others through the whole immersion time in both tap water and alkaline solution at $20^{circ}C$ and $80^{circ}C$.