The　derivations　of　the　center　of　the　ball　installed　on　the　rotary　table　are　measured　by　double　ballbar　with　circular　paths　in　the　error　sensitive　directions.　Hence,　there　are　nine　results　measured　from　three　mounting　positions　of　the　ball　at　the　same　rotation　angle,　to　form　four　axis　location　and　six　motion　errors　identification　model　using　homogeneous　transformation.　In　addition,　the　condition　number　is　applied　to　select　the　optimum　installation　parameters　of　the　ballbar.　Finally,　the　measurement　and　verification　experiments　are　conducted　on　the　four-axis　machining　center.　All　the　elements　of　the　geometric　error　can　be　identified　by　the　proposed　method　from　the　measured　results　with　high　accuracy　and　measuring　effectively.　Then,　comparing　the　predicted　value　from　the　identified　errors　with　the　measured　results　by　ballbar,　it　shows　that　the　absolute　residual　error　is　less　than　0.003　mm.