To record local earthquakes, the telemetric-digital seismic network of the Tabriz comprising of eight three component seismic stations was installed in the north-west of Iran. This array started operation at the end of 1995. Investigation of archived data in the Tabriz seismic network between 1999 to September 2004 shows that magnitude values in the database have not been determined using a single formula or a specific method. Comparison of magnitude values in the database with those values that have been published in the ISC/NEIC bulletin shows that magnitude values calculated in the Tabriz network are underestimated for events occurred in greater distances. By using magnitude values in the database and corresponding mblSC/NETC (mbISC or mbNETC) values, a scale as M log (v/4?)+2.6log (?)-2.2 is derived for determination of the magnitude in distance range of 170 to 1000 km, where v is peak-to-peak amplitude in micrometer/second and A is epicentral distance in kilometers. This formula gives a better estimate for the magnitude of events in comparison with the formula that is presently used in the Tabriz seismic network.