A number of peak-to-average power ratio (PAPR) reduction techniques have been proposed for orthogonal frequency division multiplexing (OFDM) systems; however, most of them require transmitting side information and thus reduce the bandwidth efficiency. The existing constellation extension (CE) schemes do not need to transmit side information. In this paper, we propose an improved CE scheme for PAPR reduction of OFDM signals with 16-QAM modulation. The proposed CE scheme has four extendable constellation points, the four corners in the 16-QAM constellation, and each extendable constellation point has two possible extended constellation points. The proposed CE scheme, when compared with previous related CE schemes, has fewer extendable constellation points and more corresponding extended constellation points, resulting in better BER performance with almost the same PAPR reduction. Computer simulations are given to demonstrate the effectiveness of the proposed scheme.