K-promoted iron oxide (Fe-K) and partially Mn-substituted Fe-K catalysts prepared by the soi-gel method showed improvement of catalytic activity in the dehydrogenation of ethylbenzene to styrene. Manganese, compared to the other transition metals (Co, Ni, Cu and Zn), was found to be an effective additive element for enhancement of the catalytic activity as well as suppressing of coke deposition. The Mn-substituted Fe-K catalysts with the atomic content of Mn in Mn-Fe mixed oxides: Mn/(Mn + Fe) (between 0 and 100%) were characterized using X-ray diffraction. The largest rate formation of styrene was observed for catalyst with 20% of manganese. On the other hand, no change in activation energy was observed. The lattice strain of the spinel phase between gamma -Fe2O3 and MnFe2O4 exhibited the maximum with 20% content of Mn. The experimental data suggest that enhancement of catalytic activity by addition of the Mn is responsible for increasing of the number of active sites in the spinel phase of the catalyst.