Abstract: Mebendazole displays polymorphism. There are three polymor phic forms (A, B and C) which were identified by means of IR and X-ray diffraction techniques.Procedures have been worked out to determine the amount of the polymorphic form C in pure mebendazole and in tablets. The sample (30 mg) and internal standard (K₃Fe(CN)₆, 6 mg) were mulled in 0.25ml of mineral oil and placed between KBr plates separated by a 0.1 mm lead spacer. The spectra were recorded by the absorbance mode between 850～800cm^-1 and 2150～2050 cm^-1. Both of the base lines are the horizontal line from 2140 cm^-1 peak valley and from 842 cm^-1 peak valley.The values of theabsorbance difference were measured at 834 and 842 cm^-1, 2114 and 2140 cm^-1 respectively. The percent content of the polymorphic form C was calculated by the method of internal standard. This analytical approach is simple and the results are satisfactory.