Thermal effect of fiber lasers should be taken into account in order to achieve high power output. Research of the thermal distribution and the control measures available are of great significance for normal and stable operation of the high power fiber lasers. We review the primary reported studies related to thermal effect of the high power fiber lasers, and introduce the fiber temperature theory, impact of the thermal effect on the output properties of fiber lasers, and the approaches for thermal effect controlling. Meanwhile, their merits and drawbacks are discussed, and the development prospect is predicted.