This paper discusses the calibration function used in the formula for determining local magnitude, ML, of earthquakes and the corrections for station site, instrument component, and the azimuth of the station with respect to the epicenter, on the basis of the data from 395 earthquakes (1.0ML6.2; 10 km△600km) recorded by the Beijing Telemetered Seismic Network during the period of 1978 to 1984. The research also stresses the different methods for the determination of magnitude MD by signal duration and the associated errors. The results indicate that the magnitude estimation can have a higher precision by using the method proposed in this paper. With signal duration data from only a few stations or even a single station, an unbiased estimate of magnitude can be obtained without any additional correction. It must be pointed out that the signal duration magnitude, MD, shows a tendency of saturation when the magnitude is greater than 4.8. In that case satisfactory results may be obtained by using another fitting equation with a greater slope.