Abstract: The importance of the loaded Q are analyzed on the basis of phase noise equation of Leeson model in feedback oscillator, the formula of loaded Q is derived by the analysis of Pierce oscillator circuit, and we can draw a conclusion that the larger C1 is, the higher loaded Q becomes. Based on this conclusion, a design of the prototype 120 MHz AT-cut crystal oscillator is presented and the experiments are carried out. The measurement results of phase noise are ?130 dBc/Hz@100 Hz, ?142 dBc/Hz@220 Hz, ?155 dBc/Hz@1 kHz and the phase noise floor better than ?175 dBc/Hz. Experimental results show that it is feasible to design low phase noise crystal oscillators based on improving the loaded Q.