The　calculation　of　roundness　deviation　is　mandatory　for　the　quality　inspection　of　rotational　parts.　How　to　evaluate　roundness　in　coordinate　metrology　is　a　key　problem　in　measurement.　In　this　paper,　a　minimum　circumscribed　circle　roundness　evaluation　method　based　on　mechanical　thinking　is　proposed.　Firstly,　the　convex　hull　of　the　data　set　probed　by　a　coordinate　measuring　machine　is　calculated　to　eliminate　the　data　that　would　not　contribute　to　the　evaluation　result.　Then　the　circle　enclosed　all　the　data　shrinks,　as　if　a　force　system　was　acted,　until　it　is　in　equilibrium.　The　problem　of　circle　shrinking　is　solved　by　calculating　the　midperpendiculars　and　intersections　of　the　line　segments.　A　program　was　developed　using　VC++　6.0　to　verify　the　method.　Two　sets　of　data　in　the　literature　were　used,　and　the　evaluation　results　verify　the　correctness　of　the　proposed　method.　The　proposed　mechanical　method　could　also　be　used　in　other　form　error　evaluations.