The R(Fe, Mo)12 (R=Nd, Y, Dy) samples have been prepared and measure d by x-ray diffraction at different temperatures. The crystal lattice constant s and degree of the thermal expansion along the different axes are calculated. As a result, we consider that the abnormally thermal expansion of R(Fe, Mo)12 compounds is mainly determined by the Fe-Fe interaction. According to structural parameters of Nd(Fe, Mo)12, the anomaly is caused not on ly by the short Fe-Fe interaction as the previous explanation on other R-Fe inte rmetallic compounds but also by the other strongly positive Fe-Fe interactions. The mechanism of the substitution by Mo affecting the thermal expansion anomal ies is also discussed.