Crystal oscillators have huge demand in medical filed especially in building medical devices. They are used in diagnostic equipment, monitoring devices, hearing aids, etc. Since they are widely used in medical devices, it is essential to maintain accurate requirements for the output phase noise of reference frequency source. At the same time it is important to find out new ways to reduce the phase noise which is the major problem in designing crystal oscillators. The importance of loaded quality factor QL is analyzed on the basis of Leeson model and the formula of QL is derived from the analysis of Colpitts oscillator circuit. According to the results, we can draw a conclusion that QL is explicitly related to circuit parameters. The phase noise of the 50MHz Colpitts crystal oscillator is simulated by the Agilent Advanced Design System (ADS). Based on the simulation results, a design of the prototype crystal oscillator is presented. The crystal resonator we use is AT-cut 3rd overtone crystal resonator with 49 U resistance welding package, its unloaded quality factor Q0 is about 1.45 × 105. The measured results of phase noise are -107 dBc/[email protected], -134 dBc/[email protected] and -152 dBc/[email protected] Experimental results show that it is feasible to design low phase noise crystal oscillators on the basis of improving QL.