The uncertainty estimation of synchrophasor measurements processes many valuable potential applications, such as detecting bad measurement data, monitoring the status of PMU setup, and providing measurement accuracy information for PMU applications. In this paper, we propose an uncertainty estimation method making use of the synchronous sampled data and the synchrophasor measurements. The fundamental wave is reconstructed with the synchrophasor measurements, and the residual signal is computed as the difference between the synchronous sampled data and the reconstructed fundamental wave. Two variables ri and ro are calculated respectively representing the power ratios of the components of the residual signal inside and outside of fundamental frequency band to the reconstructed fundamental. Theoretical analysis has proven that with reasonable selection of fundamental frequency band, ri is an approximately unbiased estimate of the measurement uncertainty, and -20log10ro is an accurate estimate of signal to noise ratio of the sampled grid signal in decibel. Experiments with simulated data and PMU recorded data have verified the effectiveness and practical functions of the algorithm.