A series of uniform microspheres of spinel-type Mn-doped ferrites (MnxFe3-xO4) with different Fe/Mn ratios were employed for the Fischer-Tropsch-to-olefins reaction (FTO). The highest selectivity of 54.4% and the space-time-yield (STY) of 73.0g.kg(cat)(1).h (1) to lower olefins (C-2-4(=)) were achieved over an Fe-Mn catalyst with 13 wt% Mn. A panoramic view of structure evolution of MnxFe3-xO4 was revealed using multiple in/ex situ techniques. We demonstrated that the expansion of lattice spacing induced by Mn exerted remarkable effects on the stabilization of MnxFe1-xO and epsilon-Fe2C during activation and reaction. Moreover, Mn endows Fe catalysts with strong surface basicity and consequently enhances the adsorption and dissociation of CO; while weakening the H availability and readsorption of olefinic intermediates. In addition, the impeded carburization of MnxFe1-xO and the reduction in particle size of catalysts jointly contribute to the volcano curve in the catalytic activity. The insights presented could guide the rational design of high performance catalysts via heteroatom doping. (C) 2019 Elsevier Inc. All rights reserved.