Microstructures of MgB2/Fe tapes fabricated by an in situ powder-in-tube method using MgH2 as a precursor powder have been studied by means of x-ray diffraction and analytical transmission electron microscopy combined with a focused ion beam microsampling technique. The overall microstructures in the tapes are characterized as densely crystallized MgB2 areas with 10–200 nm grain size, uncrystallized areas mainly containing MgO and amorphous phases enriched with B, and a number of holes and cracks. The crystallized MgB2 areas increase upon doping with SiC nanoparticles. Si and C atoms decomposed from SiC nanoparticles during heat treatment exhibit different spatial distributions: the Si atoms are inhomogeneously distributed, forming silicides such as Mg2Si with grain size of 5–20 nm, while the C atoms tend to be uniformly distributed in the MgB2 matrix. A significant difference in distribution of O atoms between the SiC-doped and non-doped specimens was observed. The processes of formation of these microstructures and their relationships with the critical current density under magnetic fields have been discussed.