In this work, the review of phase noise in voltage controlled oscillator (VCO) which is very important system in all kinds of wireless communication system is presented. Such kinds of VCOs can be fabricated by hybrid method using PCB substrate or by semiconductor technology using silicon, GaAs, InP, complementary metal oxide semiconductor (CMOS), micro-electromechanical sytems (MEMS) and so on. Recently there are many kinds of design tools like ADS, AWAR etc. for RF system design. Especially, the phase noise is essential parameter to be considered during the design and simulation process. Optimization of such parameter is also important in simulation because phase noise cannot be totally eliminated but it can be reduced using different techniques. If phase noise is maximum in the signal, spectrum purity cannot be expected, eventually, the system will be failed to extract the real signal. That means, the performance of the overall system will not be degraded by improving the VCO phase noise performance