One of the practical methods of quick estimating magnitude and epicentral distance for an earthquake is B−Δ method that is considered according to a single seismic record by using initial seconds of P-wave. This method is used in Earthquake Early Warning Systems (EEWS). The innovation of this study is determining picking P-wave arrival time automatically by using MATLAB software. By fitting a simple function as y(t)=Bt.e-At to the initial seconds of the seismic waveform envelope, coefficients A and B are obtained by the least-squares method. The results of this study show that logB has inversed relation to logΔ where Δ is the epicentral distance. By analyzing 273 vertical component accelerograms of Ahar-Varzaghan, 38 recorded data with magnitude Mw=4.5-6.2 and epicentral distance less than 90 km are processed and the relation logΔ =-0.35logB+2.05 is earned for this region and for estimating of earthquake magnitude Mest=1.83log Pmax-1.52logB+5.33 is obtained according to highest recorded amplitude initial seconds of P-wave.