In grinding practice, heterogeneous poli-mineral systems are more common than homogeneous mono-mineral system. The heterogeneity of raw materials in mineral beneficiation grinding circuit results by virtue of the varying properties of mineral constituents in an ore. Therefore, the main objective of this investigation is to study the effect of preheating on the grindability of quartz as a mono-mineral ore, and nepheline syenite as a poli-mineral one. 2n Factorial method was applied to arrange the studied parameters affecting the preheating of each of the two studied ores according to the importance of their relative effect on its grindability. The results revailed that incremented percentage of -200 µm in the preheated ground poli-mineral ore was less than that of mono-mineral one by about 32.9%. This behavior was interpreted by containing nepheline syenite on more than one mineral, consequently, phase change anisotropic thermal expansion or contraction of the grain boundries have a markedly effect on the grindability of preheated rocks, especially those contain more than one mineral.