The stress and density of cracks has increased before medium and large earthquakes and then the seismic velocity of P and S waves will change. Thus, measuring the seismic wave velocity (Vp and Vs), determining the ratio of Vp/Vs and changes this parameter versus time in a period of time before the occurrence an earthquakes can be a useful tool to investigate the probability of the earthquake forcasting. Determining the velocity of seismic waves has always been a topic of interest for researchers, but over the past two decades, extensive studies have been conducted to determine the velocity ratio of Vp/Vs before the occurrence an earthquakes as an earthquake predictor. A strong earthquake shook the city of Tazehabad and the surrounding villages in Kermanshah province on the early morning on Sunday, August 25, 2018. According to IRSC report, the magnitude of this earthquake was 5.9 at a depth of 8 km, and according to USGS report, it was 6.0 at a depth of 10 km.The goal of this study was to investigate the changes in the Vp/Vs velocity ratio before and after this earthquake. From the study of these changes, it was observed that before the occurrence of medium and large earthquakes, the value of the velocity ratio Vp/Vs decreases, sharply and then increases after the earthquake.