Abstract: The glass fiber reinforced plastic (GFRP)-concrete composite beam consists of a concrete slab on the upper side, a GFRP profile on the lower side and shear connectors to connect these two sides. A test study was conducted on the behaviors of two GFRP-concrete composite beams (non-prestressed and external prestressed composite beams each had one) under sustained load for one year. A time-dependent finite element parametric analysis of 24 GFRP-concrete composite beams, taking coupling effect of the shrinkage and creep of concrete and creep of GFRP profile into account, was carried out for 50 years. The results show that long-term deflections of the non-prestressed and external prestressed composite beams are 1.42 and 2.91 times of their respective instantaneous deflections under sustained load for one year. The long-term slips of the non-prestressed and external prestressed composite beams are 0.230 mm and 0.164 mm, respectively. Compared with initial slips, the terminal slips of the two composite beams increase by 53.3% and 58.2%, respectively. The ratios of long-term deflection to the instantaneous deflection for the non-prestressed composite beam are between 1.50-1.56 while the ratios of the long-term deflection to the instantaneous deflection for the external prestressed composite beam are between 3.03-6.08. Based on the above researches, calculation proposals of long-term deflections of GFRP-concrete composite beams were proposed.