Elastic electromagnetic form factors of nucleons are investigated both for the time-like and the space-like momentums by using the unsubtracted dispersion relation with QCD constraints. It is shown that the calculated form factors reproduce the exper
imental data reasonably well; they agree with recent experimental data for the neutron magnetic form factors for the space-like data obtained by the CLAS collaboration and are compatible with the ratio of the electric and magnetic form factors for the time-like momentum obtained by the BABAR collaboration.
