Measurement of sin2phi_1 at Belle

With 6.2 fb^-1 of data collected on the Upsilon(4S), Belle reports its first measurement of sin 2\phi_1 = 0.45^{+0.43}_{-0.44}(stat)^{+0.07}_{-0.09}(sys). The result was obtained by fitting the proper time distribution of flavor tagged and fully reconstructed neutral B mesons decays to five different charmonia plus a Kshort or Klong channels. In this paper the analysis and results will be described briefly.