Basing upon the Megaw’s model of superstructure of plagioclase (a0, 9b0, 2c0) we studied some e diffractions of this mineral and proposed for e-plagioclases (An33.3 to An72 a correction formula of e diffraction, δa+δc=160°+N°, where N is the number of increased anorthite-like subcells more calcic than An50. Therefore, δa + δc will be increased by one degree for each increased anorthite-like subcell after An50.If the proposed formula holds true, we may establish following group of equations :δa + δb = 40°δa + δc = 160° + N°where a*, b*, c*, α*,β*, γ* are reciprocal parameters of An80 with typical IT lattice.The calculated values of (δa+δc) of twenty e-plagilclases are shown in Table 1. The calculated ualues coincide well with the observed ones, and the error of mean square root is only±2°. So, it is necessary to rewrite δa+δc = 160° as δa + δc=160°+N°. An obvious deviation of e-diffraction parameters, however is noted for the plagioclases more sodic than An33.3 or more calcic than An72, which is caused by the predominance of CT or PT umits over IT units in their lattices.According to Megaw’s superstructure model, we get An%= N/18. 100, where N is the number of anorthite-like subcells in the supercells. Therefore the formula of width of anorthitelike subcell comain in the direction of b axis is given as follows:where n is the number of anorthite-like subcells in the direction of b axis, and b is the length of the cell edge in the direction of b axis of An80 with typical JT lattice =12.87A). Hence, we obtainIt can be seen from Table 1 that all the calculated values of [dtheor]. b coincide fairly well with the observed ones. Therefore, it is appropriate to take Smith’s T (periodicity of supercells) as the width of an anorthite-like domain in the supercells on b axis. Moreover, the calculated values also agree well with the observed ones in the* Smith (1974) wrote this formula as T =1/2|△s| diagrams of T (d) versus An% (see Figs., 5 and 6), with the exception of An<33.3 or An>72. We hold that the e-diffraction of plagioclase is related to the anorthite-like domains.As is well known,Hence, s + S0 + △s. When △s = 0 (or δh, δk and δ1 equal to zero), the above equation corresponds the condition of normal Bragg ’s diffraction; When s≠O, it corresponds the non-Bragg’s diffraction.Because we always get δa + δb=-40° and δa + δc = 160°+N°, as a result, we haveConsequently, the periodicity of all e-plagioclases is 9, whereas only in the case of N = 0(An44.4 - An50) that of 2δh-δl will be 9, because the latter changes with the composition of plagioclases.