A method to measure the t-quark mass at LHC with small systematic error.

A method to measure the top quark mass in the inclusive lepton plus jets channel at LHC is presented. This method is specially designed in order to reduce systematic errors of the measurement. A simulation of a tt production and an ATLAS detector response is performed. It is shown that for 10 fb^{-1} integrated luminosity the proposed method will give a statistical accuracy of the order of 50 MeV and a systematic error of the order of 360 MeV for the top quark mass estimate.