Novel Fe/MnK-CNTs nanocomposites are developed as catalysts for direct production of lower olefins from syngas, delivering a high iron time yield of 337.2 mol(CO)>s(-1) with 51.3%C selectivity toward C2C4 olefins under the optimal reaction conditions (270 degrees C, 2.0 MPa, 30,000 mLh(-1)). These catalysts are optimized by varying calcination temperature from 150 to 400 degrees C. Multiple techniques including transmission electron microscopy, Elemental mapping, X-ray diffraction, X-ray photoelectron spectroscopy, H-2-temperature-programmed reduction, and Raman were employed to reveal the relationship between the catalyst nature and unique catalytic behavior. In particular, the resultant catalyst from the calcination temperature of 220 degrees C exhibits the highest selectivity of C2C4 olefins as well as good stability, which are enabled by the trade-off among the effects of iron particle sizes, promoters, metal-support interaction and support surface chemistry. Moreover, influences of reaction temperature, reaction pressure and space velocity are also investigated. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 154-161, 2017