In order to restrain the nonlinear clipping distortion caused by the l ight emitting diode (LED) in visible light communication orthogonal frequency division multiplexing (VLC-OFDM) syste m,a modulation named polarity index modulation orthogonal frequency division multiplexing (PIM-OFDM) is proposed in this paper.The polarity and amplitude data of the bipolar direct current optical (DCO)-OFDM si gnal are carried by two blocks in PIM-OFDM system.In the first block,the positive and negative polarities of bipolar sign al are coded and transmitted by on-off keying (OOK) signal.In the second block,amplitude information is achieved by t aking the absolute value of the bipolar signal.The system performance of PIM-OFDM is almost 3dB better than th e asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) with the same con dition.With the optical power constraints,the optimal power allocation between DC bias and input signal is obt ained under the principle of maximum effective signal-to-noise ratio (SNR).Simulation results show that the effect ive SNR decreases with the increase of DC bias under the low optical power constraints.However,the effective SNR woul d increase first and then decrease with the increase of DC bias under the high optical power constraints. It is also shown by experiments that the bit error ratio of PIM-OFDM improves an order of magnitude than that o f ACO-OFDM with the same transmission rate.