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Abstract. Process of monodomain formation in FeBO 3 antiferromagnet in the static magnetic field applied in the magnetically easy plane is studied by NMR technique at the temperatures 4.2 and 77 K. It is found that the 57Fe NMR signal splits into three absorption lines. Dependence of the intensities and the resonance frequency changes of the "central" and of the "side" signals on the static field amplitude is investigated. It is shown that the observed effects are determined both by layered character of domain structure and weak ferromagnetism of iron borate. Possible utifizations of method (based on the NMR frequency differences of nuclei in different domains in the static magnetic field) for investigation of the domain structure character and the remagnetization dynamics of magnetically ordered materials are discussed.As is known, at temperatures below 348 K iron borate (FeBO3) is an antiferromagnet with weak ferromagnetism and is characterized by the nonvanishing vectors of antiferromagnetism L--M I -M 2 and total magnetization M --M a + M 2. The sublattice magnetic moments M1, M2, as well as L and M vectors lay in the easy plane (111) perpendicular to the threefold axis. According to [1,2], the structure of domains in FeBO 3 is represented by distributed over thickness layers-domains with different directions of net magnetization in the basal plane. At 77 K the boundaries between the layers are 120" Bloch domain walls S~~ [2]. In its turn, each layer has 180" domains which are separated by Neel walls S• having considerable mobility in the constant magnetic field H 0 even of the order of 10e applied in the basal plane. According to the model of domain structure formation suggested in [3], it can be assumed that at first Neel walls Sz in the layers-domains disappear, if H 0 is directed in the easy plane (111). In those domains where M vector comprises the acute angle with the H 0 direction, the rotation of M to the H 0 direction will take place. Domalns with the opposite M direction disappear due to the displacement of walls. Further increase in H 0 results in