The magnetization process of an Nd-Fe-B hydrogenation-disproportionation-desorption-recombination (HDDR) magnet was studied by measuring its initial recoil curves. When compared with those of Nd-Fe-B sintered, HDDR and melt-spun magnets, the behavior of the initial recoil curves of the HDDR magnet was similar to that of the sintered magnet when the applied magnetic field was relatively small. The behavior of the initial recoil curves of the HDDR magnet became similar to that of the melt-spun magnet when the applied field became larger. The magnetic microstructure of the HDDR magnet was observed by means of a magnetic force microscope (MFM). When the HDDR magnet was thermally demagnetized, the distance of two domain walls was smaller than the average grain size of the HDDR magnet. On the other hand, the distance of two domain walls was larger than the average grain size of the HDDR magnet when the magnet was magnetically demagnetized. These results suggest that the HDDR magnet contains a multi-domain structure when the magnet is thermally demagnetized or when the applied field is relatively small. However, the HDDR magnet has only a single-domain structure after the application of sufficiently large fields.