We discussed the mode coupling in an optical fiber tapered coupler with itsrefractive index gradually changins along its propagation axis caused by photo-inducedrefractive effect. Under scalar approximation, the field in the stead index fiber andparabolic-index fiber tapers is expressed in local normal modes to analyze the coupling of thefundamental mode to its reflected one in opposite direction and high-order ones in codirection. The numeric resultS for two different taper profiles in step-index fiber show that the couplings to the reflected mode and the high-order modes are relatively small provided the change is gradual, and the fundamental mode has the capability of excellent transmission which can adjust itself adiabatically to the reflective index change.