Abstarct: Nowadays due to the expansion of wireless communications, improvement of the quality of the transmitter and receiver systems, is an important issue. Oscillators are one of the main parts of the telecommunication system. Among the features of an oscillator its phase noise has a significant role in the quality of data and signals sent and received by the transceiver systems. Thereby reducing oscillator phase noise have long been the subject of many studies. Several methods have been proposed to calculate the phase noise of which Hajimiri method is highly regarded. Among the different structures of the oscillators, CMOS cross-coupled oscillator is one of the structures that have good phase noise characteristic. In previous works the study of the effect of capacitor CT in parallel with the tail current source on cross-coupled oscillator phase noise performance has been noteworthy. In a typical cross-coupled oscillator, this capacitor is the parasitic capacitor of the tail current source and analysis and understanding of its effect on the oscillator phase noise behavior leads to interesting results. In a study using analytical equations as well as simulation and manufacturing, it is shown that capacitor CT has noise rejection property on the one hand and on the other hand reduces the quality factor of LC circuit. In several studies using simulation and manufacturing has been proven that by putting a large capacitor in parallel with the current source, the current source noise can be filtered out. In this thesis the effect of the capacitor CT in the more general case has been discussed using analytical equations and calculating the impulse sensitivity function that is the baxse of the Hajimiri’s method in analysis of phase noise. To evaluate the accuracy of calculations, the results of mathematical analysis were compared with the results of the simulations. baxsed on the values and formulas presented in this thesis, the phenomenon of reducing oscillator's output phase noise by increasing the capacitor CT is justified by analytical values. Also one of the very interesting results obtained from the equations is that capacitor CT greater than a certain value, will have little effect on phase noise reduction.