Abstract: Through analyzing some measurement procedures of refractiveindex profile of singlemode and multimode fibers; their essential differences are obtained. Since the core of singlemode fiber is very small, its propagation mechanism can be analyzed only by the wave theory,and the measurements with the farfield and nearfield methods are all based on the scalar Helmholtz wave equation, that is the refractiveindex profile measurements for singlemode fiber are according to its basic transmission theory. Whereas the core of multimode fiber is larger, the ray theory is appropriate to analysis of its propagation principle. The reason is that the multimode fiber refractedray method and nearfield scanning method are all based on the numerical apertures of various radii and the corresponding refractive patterns and propagation patterns.