Abstract: In a baseband direct-detected optical orthogonal frequency division multiplexing (DDO-OFDM) system, when the total transmitted power is constant, increasing the power of pilots can improve channel estimation accuracy but also increases the interference from them. Therefore, in order to obtain a best performance, the optimal ratio of pilot-to-total power should be given. In this letter, we compared different systems with different pilot power to total power ratios by simulations. The results showed that the power allocation with an optimal pilot power to total power ratio was found and the optimal power allocation obtained the most accurate channel estimation and the highest receiver sensitivity.