Abstract: The thermal effect of high power fiber lasers is studied.Based on an analysis of the thermal effect,a model of steady state temperature distribution of double cladding fiber laser is established.The axial and radial temperature distributions are numerically simulated,and the relation between the center temperature at the fiber facet and the output power are obtained for different fiber lengths,radii of cross section and cooling conditions.The simulated results showed that axial cooling of the fiber end regions can significantly reduce the influence of thermal effect of the active fiber in a high power single-core system.