The optimal transmitters(TXs) selection method is proposed to operate at maximum power efficiency in a wireless power transfer(WPT) system using multi-TX. The proposed method is determined by the ratio of the coupling coefficients between TXs and RX and is not affected by other parameter. Moreover, the proposed criteria are generalized so that they can be applied according to the number of TXs. The power of 100W is transferred at 87.2% and 76.7% with and without the proposed method, respectively.