This　paper　focuses　on　the　design　of　a　milling　cutter　for　machining　the　arc-cycloidal　helical　rotor　of　a　Roots　blower.　The　milling　cutter　is　developed　on　the　basis　of　a　novel　rotor　of　a　Roots　blower　which　consists　of　both　concave　and　convex　arcs　and　a　cycloidal　curve　in　the　transversal　section.　The　geometric　analysis　of　the　transverse　section　and　the　helical　tooth　profile　for　the　proposed　rotor　are　presented.　The　paper　reveals　the　contact　characteristics　between　the　cutter　and　the　rotor　blank　being　machined　and　derives　the　normal　vectors　of　three　different　tooth　sections.　The　paper　further　develops　equations　of　the　contact　line　for　the　conjugate　motion　of　the　cutter　and　the　rotor　blank　and　presents　a　design　methodology　of　the　milling　cutter.　The　interference　in　the　machining　process　is　then　investigated　and　numerical　results　are　produced.　The　results　from　the　analysis　demonstrate　that　there　is　no　interference　in　the　machining　using　the　developed　cutter.　The　developed　cutter　is　then　demonstrated　in　the　end　of　this　paper　in　machining　the　novel　arc-cycloidal　helical　rotor　with　three　lobes.　©　IMechE　2004.