The popular evaluation of wear measures volume loss or weight lose. In this paper, in order to study the wear phenomenon from the view point of the generation of wear particles, the number of wear particles and the size distribution were measured by the SEM-tribosystem. The results of this experiment are summarized in the following two points. The number of wear particles was not only affected by the quantity of wear loss but also by the size of wear particles. The size distribution of wear particles could be well described by exponential and logarithmic distribution. In addition, the theory was introduced to estimate the value of the total number of wear particles. The number of wear particles estimated by the proposed theory agreed well with the experimental value.