Accurate power system parameters are the foundation to form a reliable power system model， which is essential for state estimation， power flow and contingency analysis， network losses analysis， stability analysis and protective relay setting calculation. Based on uniform transmission line equations andΠ-type equivalent circuit equations， and on the assumption that voltage phasors and current phasors at both ends of a transmission line are known， the methods to calculate transmission line distributed parameters and accurateΠ-type equivalent circuit parameters are given. Based on phasor measurement unit（PMU） measurements and statistical theory of normal distribution， an algorithm for on-line estimation of transmission line parameters is presented. Examples based on wide area measurement system（WAMS） in North China Power Grid show that the proposed methods are accurate and reliable.