Westbrook's theory has been adopted to analyse.the effects of a grating monochramator on the measurement of modal reflectivity at AR coated facets of a semiconductor laser diode. It has been verified that the measured modulation index and reflectivity changes with the exit slit width periodically. The period has been identified to correspond to the diode mode spacing. The analysis has also predicted that when the passband width of the monochramator is somewhat larger than the diode mode spacing, the measured peaks and valleys will be different from their real positions by a half of the mode spacing.