Abstract: In the hadronic part of semi-electronic ttbar decay channel, t->Wb->jjb, both W boson and top quark masses are constrained to agree with their measured values. An event-by-event Kinematic Fit using Lagrange multipliers is used to enforce the constraints. Residula corrections on the energy scale of both light jets from W boson decay and b jet from top quark decay are estimated by minimizing a chi2 distribution coming out of the fitting procedure.