The advantage of the definition for m. f. d （mode field diameter） given by K. Petermann is stated in this paper

and a far-field mask measuring system established accrding to the definition is introduced. Considering the practical transparence of different areas on a mask plate

we have modified the calculation formula of m. f. d given in some foreign literatures. This system can easily be applied to determine m. f. d of monomode fibres at the low-loss wavelength windows of 1.3μm and 1.55μm by means of the formula derived in this paper.