Mode coupling, bend distortion and bend loss are important considerations in large mode area (LMA) fiber laser. Both mode coupling and mode distortion in the step-index fiber are studied theoretically in order to gain the effect of mode distortion on the beam quality. The dependences of mode power on propagation distance, effective bend radius are calculated with the bend radius experiencing sudden and gradual transition. Numerical simulations show that, if the distorted fundamental mode experiences smooth transition from bend fiber to straight fiber, the excitation of higher order modes could be ignored when the bend radius is larger than 3 cm. But the bend radius must be larger than 20 cm when the mode experiences sudden transition.