Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is a low frequency radar. The MARSIS wavelength is about 50-100 m. These radar waves penetrate deeply in the Martian ground and the first radar echo is due to few tens meters of the surface. The amplitude of this surface echo provides useful information on the shallow subsurface. Using MARSIS radar data, we extract the reflectivity of the Martian surface from the radargrams and then we build a global radar reflectivity map. We will describe our method for the extraction and the calibration of the reflectivity. In this calibration, we correct the absorption due to the two-way of radar waves through the Martian ionosphere and compensate the surface roughness effect. The surface roughness effect is estimated by simulating the radar surface returns for each MARSIS orbit. Finally, we will present a reflectivity map without roughness effect and discuss the reflectivity variations due to change in the dielectric constant.