The required bit resolution of digital-analog-converter (DAC) for optical OFDM was evaluated through simulation and experiment. We found that the signal degradation caused by quantization depends on the number of subcarriers theoretically, but the other noise generated in experiment suppresses the dependency. Additionally, signal degradation caused by quantization is independent of subcarrier modulation, and the performance improvement saturates with increasing bit resolution. With the criteria for the BER of 1×10^-3, the required DAC resolution by simulation with ideal analog-digital-converter at receiver is 3bits for QPSK, 4bits for 8-QAM and 5bits for 16-QAM and 32-QAM. From the experimental result, the required bit resolution is increased as 6bits for 32-QAM.