The aim of this research paper is to derivation a mathematical equation on Tripoli City to measurement the rain fall rate (R mm/h) on any geographical area in the range of the scan according to the used radar and that will be by using a reflectivity which comes to the radar weather station (Reflectivity (Z)) and make a comparison between this equation , Marshall and Palmer equation and rain fall rate measuring by rain gauges of weather stations close the radar. These are four weather stations distributed on (4) geographical areas in Tripoli City. These are four rain gages used to account the rain fall rate with traditional ways, and comparison between the rain fall rate which measured by weather radar station (Rr1) by using the equation of Marshall and Palmer, and derivative equation (Rr2) , and calculate Standard deviation (S.D) to the difference between the rain fall rate measured by weather radar and weather stations (Rr - Rg) for both cases, make a comparison of the results in range of radius circle (80 km), and obtaining the special equation of Tripoli City. Key Words : Weather radar, Rain Gauges, Precipitation, Reflectivity of radar, Metrological weather station.