Measurement of asymmetry in polarized electron-nucleon elastic scattering provides information on the weak charge and current distributions in the nucleon through the interference between photon and Z sup 0 exchange graphs. This asymmetry is due to the parity violation in the weak interaction. The expansion of weak form factors in term of flavor (u,d,s) form factors leads to information on quark structure of the nucleon, especially on the strangeness contribution to the vector form factors. The measurement of the parity violating asymmetry in elastic scattering of polarized electrons on protons at MAMI facility in Mainz is presented. At the MAMI current maximum energy of 0.855 GeV, the electrons scattered out a 10 cm liquid hydrogen target are detected between 30deg and 40deg. Under these kinematic conditions, the value of the transfer momentum Q sup 2 , weighted by the elastic cross section in this angular range, is Q sup 2 =0.233 (GeV/c) sup 2. This experiment measures the G sub E sup s + 0.218G sub M sup s combination of the electric and magnetic strange form factors. The detection is done using a calorimeter built of 511 Cerenkov crystals in the phase I of the experiment. The experimental set-up with its main characteristics are shown. The origin and magnitude of possible systematic and helicity correlated false asymmetries are discussed.