It is of the key importance to choose the parameter s/r for the design of the light coupling from a laser diode into a multimode fiber with spheric end. This paper discusses a method by which the optimal radious of curvature of the spheric-lensed fiber roptim and the optimal distance from the endface of the fiber to the light source soptim can be rather easily found. In this method the analytic expressions of γmin and （y/r） γmin are derived because y/r is adopted as the variable

therefore

the parameter （s/r）optim can be determined precisely.