We studied the influence of Cu doping in Mn sites of single phase bulk materials La2/3Ca1/3Mn1-xCuxOy on their magnetoresistance effect at liquid nitrogen in experiment. We find that, for the best doped sample (x≈0.15), it shows great magnetoresistance effect clearly. Moreover, the main variation of the magnetoresistance is under the field below 1T. The MR ratio rises up to ～5×104% when the field is increased to 6T. It increases three orders of magnitude compared with samples deviated from the best doping.