In　order　to　estimate　the　state　of　charge　(SOC)　of　battery　pack　in　hybrid　electric　vehicle　accurately,　and　eliminate　the　accumulated　error　of　Ampere-hour　method,　a　SOC　estimation　algorithm　using　Kalman　filter　technique　is　developed　First　the　equivalent　circuit　model　of　the　battery　pack　is　founded,　and　then　the　dynamic　state　and　measurement　equations　are　established.　By　processing　the　current　and　voltage　data　of　the　battery　pack　collected　from　the　real　vehicle　test,　the　algorithm　using　Kalman　filter　technique　can　estimate　the　open　circuit　voltage　of　the　battery　pack　effectively,　and　then　SOC　of　the　battery　pack　can　be　calculated.　It　indicates　that　the　algorithm　proposed　has　much　potential　in　the　SOC　estimation　in　HEVs　through　experiments.