Magnetic domains in fine particles of Nd2Fe14B ball-milled to less than several micrometres in size have been studied by the colloid-scanning electron microscope (SEM) method, which is a method for observation of small domains. The single-domain state has been clearly found on thecplane in several particles. Their domain patterns are shown for two single-domain particles of 1·0 and 1·2 μm size. These particle sizes are compared with a simple theoretical estimate.