Axial　field　flux-modulation　permanent　magnet　(AF-FMPM)　machines　have　been　developed　for　direct　drive　applications　such　as　wind　power　generation,　HEVs,　and　railway　traction.　However,　the　existing　studies　on　AF-FMPM　machines　are　limited　to　the　PM　rotor　structure,　and　less　research　has　been　carried　out　on　AF-FMPM　machines　with　PM　stator　structure.　This　paper　studies　two　axial　field　flux-modulation　permanent　magnet　(AF-FMPM)　machines,　i.e.,　NS　or　NN　type,　which　all　consist　of　the　same　stator　and　rotor　structures　but　the　dual　identical　outer　surface-mounted　PM　stators　face　each　other　in　different　position.　A　comprehensive　theoretical　analysis　and　global　optimization　of　the　AF-FMPM　machine　based　on　ANSYS　Maxwell　3-D　FEA　is　presented.　The　performance　comparisons　between　the　AF-FMPM　machine　(NS)　and　AF-FMPM　machine　(NN)　are　presented,　including　the　influence　of　the　critical　structural　dimensions　on　the　machine　performance,　phase　flux　linkage,　phase　back　EMF,　cogging　torque,　and　average　torque　performance.　The　results　show　that　the　phase　back　EMF　and　average　torque　of　the　AF-FMPM　machine　(NS)　are　more　sinusoidal　and　higher　than　that　of　the　AF-FMPM　machine　(NN).　Furthermore,　the　NS　type　machine　shows　the　non-saliency　characteristics,　while　the　NN　type　machine　is　the　salient　machine.