Synchronized phasor measurements during dynamics tend to be affected by prevailing system frequency. Some major blackouts in power systems are indeed featured with very large frequency disturbance. Quantitative study done in this dissertation shows that small frequency disturbance may lead to measurement errors, and large frequency disturbance may lead to wrong measurements as well as catastrophic results if applied in system protection and control. The purpose of this dissertation is to bring up this issue, point to some possible solutions and application examples. A synchronized frequency measurement method, which has better dynamic performance, is proposed in this dissertation. Based on this accurate synchronized frequency, a phasor compensation algorithm is proposed to correct the errors due to frequency disturbance in legacy PMUs or as alternative frequency tracking algorithm in new PMUs. Phasor positioning and unbalance issues are also investigated in this dissertation. With these improved synchronized measurements, wide area protection and control can be achieved with higher reliability. As an application example, traditional preset out-of-step protection could be replaced by the adaptive out-of-step protection using wide area measurements. Real-time swing curve and real-time EEAC based adaptive out-of-step protection schemes are developed respectively in this dissertation. Numerical Simulations are performed for validation of the proposed concepts. / Ph. D.