To　meet　the　high-precision　positioning　requirements　for　hybrid　machining　units,　this　article　presents　a　geometric　error　modeling　and　source　error　identification　methodology　for　a　serial-parallel　hybrid　kinematic　machining　unit　(HKMU)　with　five　axis.　A　minimal　kinematic　error　modeling　of　the　serial-parallel　HKMU　is　established　with　screw-based　method　after　elimination　of　redundant　errors.　A　set　of　composite　error　indices　is　formulated　to　describe　the　terminal　accuracy　distribution　characteristics　in　a　quantitative　manner.　A　modified　projection　method　is　proposed　to　determine　the　actual　compensable　and　noncompensable　source　errors　of　the　HKMU　by　identifying　such　transformable　source　errors.　Based　on　this,　the　error　compensation　and　comparison　analysis　are　carried　out　on　the　exemplary　HKMU　to　numerically　verify　the　effectiveness　of　the　proposed　modified　projection　method.　The　geometric　error　evaluations　reveal　that　the　parallel　module　has　a　larger　impacts　on　the　terminal　accuracy　of　the　platform　of　the　HKMU　than　the　serial　module.　The　error　compensation　results　manifest　that　the　modified　projection　method　can　find　additional　compensable　source　errors　and　significantly　reduce　the　average　and　maximum　values　of　geometric　errors　of　the　HKMU.　Hence,　the　proposed　methodology　can　be　applied　to　improve　the　accuracy　of　kinematic　calibration　of　the　compensable　source　errors　and　can　reduce　the　difficulty　and　workload　of　tolerance　design　for　noncompensable　source　errors　of　such　serial-parallel　hybrid　mechanism.　©　The　Author(s),　2024.　Published　by　Cambridge　University　Press.