Optoelectronic Oscillator(OEO) can generate low phase noise microwave signals. In an OEO, the Mach-Zehnder Modulator(MZM) can work either at the quadrature-biased point to minimize the fundamental oscillating signal loss or at the Null-Transmission Point(NTP) to obtain the frequency-doubling signal. In the OEO where the MZM is NTP-biased(NTP-biased OEO), the fundamental signal can be obtained by using an electrical frequency divider so as to maintain oscillation in the loop. In this paper, the phase noise performance of the two OEOs(Q-biased OEO and NTP-biased OEO) is analyzed theoretically. According to the theory, it can be seen that the phase noise performance of the NTP-biased OEO is better than that of the Q-biased OEO. With the simulations, the phase noise floor of the NTP-biased OEO is 3 dB better than that of the Q-biased OEO. Besides, the free spectral range will not be affected by the frequency division in the NTP-biased OEO.