Abstract: Discusses theoretically the relation between BER and PAPR in an OFDM system which has been processed to decrease PAPR. On conditions that the power amplifier efficiency is ensured, there is a PAPR that enables the BER to be minimum. Such a PAPR is defined as an ideal one but uncertainly a minimum one. What is mentioned above is demonstrated by two existing methods to reduce PAPR, i.e., wave clipping and selective mapping. Taking the former as example that is used to reduce PAPR in an OFDM system, the software MATLAB is applied to simulating the system and the results verify the correctness of the demonstration: the ideal PAPR is available to find out with less BER to improve the system performance.