The application of polymer gears in power transmission has been considered recently due to their advantages over metal gears, e.g., lighter weights and lower costs. Much further research work needs to be carried out to understand their wear mechanisms and to establish their design methods. This paper aims to investigate the wear and failure behavior of the poly-ether-ether-ketone (PEEK) gear against the steel gear under oil jet lubrication through the durability test with the standard FZG test rig. The service life and wear loss of PEEK gears were experimentally recorded under different moderate loading conditions ranging from the output torque of 10–20 Nm with a fixed input rotational speed of 1000 rpm. Moreover, the tooth surface morphologies and the wear loss of PEEK gears under different running stages were investigated. The experimental measurements reveal that there is a critical load magnitude for the transition of wear performance. With a load above 15 Nm, the PEEK gear wear rate increases rapidly, leading to a quick breakage failure, whereas the gear wear rate with a torque below 15 Nm is much lower. All tested PEEK gears finally failed with a pitting-induced tooth breakage mode under moderate loading conditions.