Two parameters for measuring the structural strain, the effective Debye-Waller parameter, B-eff, and lattice strain, epsilon, were evaluated on a natural pyrite (FeS2) after grinding. The effective Debye-Waller parameter, B-eff, which depends on the displacement of atoms in the crystal, was calculated for the overall crystal, B-eff(FeS2), and for sulphur, B-eff(S), from the intensities of the X-ray diffraction lines. The B-eff(S) increased markedly with increasing grinding time, while B-eff(FeS2) did not change significantly. The lattice strain, epsilon, was not recognized. These observations suggest that the displacement of sulphur atoms preferentially takes place by grinding. The relations between these B-eff values and crystallite size, L, were observed to be common for two different methods of grinding. This tendency was considered to be an inherent property of pyrite. The value of B-eff(S) is a useful index to estimate mechanically caused strain in pyrite.