In order to improve the spectral effic iency of asymmetrically clipped optical orthogonal frequency division multiplexi ng (ACO-OFDM),a unipolar asymmetrically clipped optical OFDM (UACO-OFDM) modulation scheme is proposed in this paper.The UACO-OFDM signal is composed of the ACO-OFDM and unipolar OFDM (U-OFDM) signals,and the signals with di fferent lengths are superposed according to the certain rules.Therefore,the superposed signal is tr ansmitted through the light emitting diodes (LEDs) at the transmitter,and only one photodiode (P D) receives the light signal at the receiver.The interference cancellation method is employed t o demodulate different signals successively at the receiver.When the modulation orders of eve ry system are the same, the spectral efficiency of UACO-OFDM is higher than that of ACO-OFDM.The bit error ratio (BER) and peak-to-average power ratio (PAPR) performance of UACO-OFDM and other traditional optical OFDM modulation schemes are compared and analyzed under the same spectral effici ency. The simulation results show that the BER performance of UACO-DFDM is better than those of other techniques.Compared with spectral and energy efficient OFDM (SEE-OFDM),when the subcarrier number is 64,the PAPR performances are very similar.But when the subcarrier numbers are 256and 1024, PAPR values of the UACO-OFDM are at least 2dB and 5dB bett er than that of SEE-OFDM,respectively.