Abstract: A new agglomeration parameter Na(50) defined as the average primary particle number included in each agglomerate is proposed. With the application of characterizing the saute of agglomeration of ultrafine ZrO₂ (Y₂O₃) powders prepared by co-precipitation, a comparison between Na(50) and the currently used agglomeration parameters AF(50) as well as AF*(50) is made. The resultS show that the parameter Na(50) is better in representing the true state of agglomeration of poWders. In addition,the relations between the average Stokes diameter and the relative density of the agglomeration has been inferred.