The durability of plain woven glass fiber reinforced plastics (GFRP) laminate in the off-axis direction in hydrothermal environment is investigated in this paper. The durability discussed in this paper are as follows 1) the degradation of the mechanical properties after immersion into deionized water, and 2) delayed fracture under constant tensile load in deionized water. The mechanical properties of GFRP laminate in deionized water were smaller compared to that in air. Moreover, the mechanical properties decreased with immersion into deionized water. The strain and its growth rate of GFRP laminate under constant tensile load increased with immersion into deionized water and moreover delayed fracture occurred faster in deionized water than in air under equal applied stress. In addition, it was suggested from the fracture surface observations that these decreases of the durability occurred after the diffusion of deionized water throughout the thickness direction.