To　accurately　estimate　the　state-of-charge　of　battery　pack　in　hybrid　electric　vehicles,　to　reduce　the　experimental　work　before　application　of　the　ampere　hour　counting　method,　and　to　eliminate　the　accumulation　of　errors,　the　Kalman　filter　is　developed　to　application.　Based　on　the　equivalent　circuit　model　of　the　battery　pack,　the　dynamic　state　and　measurement　equations　are　established,　by　processing　the　data　collected　from　the　current　and　voltage　of　battery　pack　in　real　vehicle　testing.　The　algorithm　developed　by　using　Kalman　filter　can　estimate　the　open　circuit　voltage　of　battery　pack,　then　state　of　charge　can　be　calculated.　It　indicates　that　Kalman　filter　has　potential　for　estimating　state　of　charge　of　batteries　in　electric　vehicles.