FeCo thin films were prepared on MgO(100), MgO(110), and MgO(111) single-crystal substrates by UHV-MBE. The effects of substrate orientation and substrate temperature on the film growth structure and the magnetic properties were investigated. FeCo thin films with (100)_bcc, (211)_bcc, and (110)_bcc planes parallel to the substrate surface grew epitaxially on respective MgO substrates. FeCo(100)_bcc single-crystal films were obtained on MgO(100) substrates. FeCo(211)_bcc bi-crystalline films grew epitaxially on MgO(110) substrates with two types of domains whose orientations were rotated around the film normal by 180 degrees with respect to each other. FeCo(110)_bcc thin films grew epitaxially on MgO(111) substrates with two type variants namely, the Nishiyama-Wasserman and Kurdjumov-Sachs relationships. Atomically sharp boundaries were recognized between the FeCo thin films and the MgO substrates, where misfit dislocations were introduced in the FeCo thin films, presumably to decrease the lattice misfits. A 0.3% reduction of the lattice spacing normal to the substrate surface was observed for an FeCo thin film prepared at a substrate temperature of 100 °C. The lattice spacing reduction decreased with increasing substrate temperature. The magnetic properties of FeCo thin films grown on MgO substrates were influenced by the magnetocrystalline anisotropy and the shape anisotropy caused by the roughness of FeCo islands which form the FeCo thin films.