To record the local earthquakes, Tehran digital seismic network comprises 12 three-component seismic stations installed in southern margin of Alborz. This array started its operation in January 1995. In the central office of this array the extracted data after processing of recorded waveforms are archived similar to other seismological centers. The archived database contains information on date of shocks, origin time, location coordinates, magnitude as well as station information such as epicentral distance and amplitude of measured P-wave. In this article, the completeness of database and the bias in the magnitude values which have been determined in this array are investigated. Investigation of database showed that some essential station information such as epicentral distance and amplitude of measured phase, which are necessary for magnitude determination, have not been completely stored in database especially in the period of 1997 to 2000. Therefore, recovering these parameters for any purpose requires analyzing waveform data. Investigation of archived magnitude values showed that magnitude values in the database have not been calculated by using a single formula or a specific method. Comparison of magnitude values in database with those values that have been published in ISC and NEIC Bulletins shows that in Tehran network, magnitude values are underestimated. By using local magnitude values and corresponding mt’ values, ML scale is calibrated to mb scale and the formula ML = log(V / 4r) + 2.5 log(A) — 1.8 is derived. For distances grater than 106 km, using this formula gives a better estimate for the magnitude of earthquake in comparison with the formula that currently is being used in this array.