Abstract: Concentrative and continuous activity of earthquakes (4.0ML6.0) occurred within a few years before and around the epicenters of the 10 coming earthquakes of ML5.0 in North China (30-42N,109-125E) since 1970,and there seemed to be a relationship between the scale of activity of these earthquakes and the magnitude of the main shock. These earthquakes are called moderate foreshocks in this study. Three parameters indicating the scale of activity of foreshocks,i.e.,energy release,duration time and area of activity are defined and the relationship between the three parameters and the magnitude of the main shock is established through regression analysis.