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This Paper proposes a new algorithm for estimating the zero sequence impedance of a live transmission line by means of Phasor Measurement Units (PMU). In this algorithm the line impedance matrix is calculated and its sequence impedance matrix calculates by means of symmetrical components matrix. A case study contains a 230kv single circuit un-transposed transmission line was simulated and results are demonstrated. One of the main advantages of this algorithm is the ability of estimating the parameters in live or on-line condition which enables the system operators and adaptive protection systems to update their parameters online without needs to disconnect or short circuit the line. Results show the algorithm capability in estimating the short and medium transmission lines zero and positive sequence impedances. In a long transmission line the estimation error increases but limited to smaller than 20% of the real value.