The ability to estimate the magnitude using the first few seconds of the P wave offers a methodology for earthquake early warning. This study introduces a new result to estimate an earthquake’s magnitude using only the first few seconds of P-wave data for use in earthquake early warning systems. We tested the maximum predominant period of the P-wave (τp max) and apply our algorithms to 57 waveforms (3.5<M<5.7, Δ<80 km) from the Kahak earthquake, M 5.7 (June 18, 2007), about 12 kilometers south of Qom near the faults trends with a northwest-southeast. In the τp max based method τp is measured through acceleration and velocity time series within the initial seconds of receiving P-waves (3-4 s). The empirical relationship between the logarithm of τp max within the initial 4 s of a waveform and magnitude was obtained as M=0.65Log(τp max)+5.3±0.4. The sensitivity of the predominant period observation to magnitude can be optimized using various frequency bands for different magnitude ranges and in different regions.