Many different methods for size measurement are known and since they differ in their physical principle, also different results are obtained. In recent past, there was a rapid development in new measurement techniques and at the present we can quickly and routine determine particle sizes in the very fine range by wet - as well as dry measurement technique. The smaller particle size, on the other side tends to increase the probability of agglomeration. The measurement of the particle sizes for magnetic materials is not very well understood and in this paper we would like to present our research results regarding the size determination of very fine magnetic materials. Because of their magnetic dipole moment these particles tend to agglomerate even more which in turn causes cartain difficulties during the measurements. Therefore it is important to know under what measurement conditions the actual particle size and not the size of the agglomerates is obtained. By choosing various materials with different specific magnetic susceptibility and by comparison ofthe analyses, carried out in various analyzers operating in different principles, we have found that by correct selection of measuring conditions and sample preparation, we are able to obtain very good results.