A theoretical model of high power multimode fiber oscillator is built based on rate equations of the multimode Yb3+ doped fiber considering improvement of the mode coupling in the core. The effect of Yb3+ dopant profiles in the core on the output characteristics of fiber oscillator is carried out. The output characteristics of the high power multimode fiber oscillator with flat and parabolic dopant profiles fibers are described respectively. The output characteristics of the fiber oscillator with 75% flat dopant profiles fibers in different coupling coefficients are described. The results show that the different Yb3+ dopant profiles may have different impacts on the output characteristics. Specially, the fundamental mode output can be achieved by designing and controlling the Yb3+ dopant profiles.