The microstructure of natural magnetite found in the Panzhihua mountain in China was studied by means of high-resolution transmission electron microscopy (HRTEM). Electron diffraction patterns show lattice modulations along the [110] direction of wüstite co-existing with magnetite. Such a one-dimensional superstructure, with a lattice parameter of 2.6a₀, is the characteristic feature of an incommensurate structure. It is related to the nonstoichiometric phase P' of Fe_0.920O that results from a deficiency of Fe atoms in the wüstite. The structural images exhibit the intergrowth of the magnetite matrix and the wüstite. The wavelike ultra-fine structure in wüstite suggests that the superstructure is possibly coincident with the displacive lattice modulation. Basal and nonbasal domains, which result from the random distribution of Fe atoms during the initial stage of nucleation of magnetite during the geological aging of wüstite, are also revealed in the structural images and are discussed on the basis of the structure of magnetite.