In　this　article,　attempts　are　made　to　study　the　aerodynamic　performance　of　co-axial　rotors　with　different　rotor　spacings　in　hover.　A　custom-designed　experimental　platform　with　seven　rotor　spacings　(z/D　=　0.16,　0.19,　0.23,　0.26,　0.29,　0.33　and　0.38)　is　applied　to　measure　the　hover　performance,　i.e.　co-axial　thrust　and　power　consumption,　and　to　optimize　the　aerodynamic　configuration　of　the　co-axial　system.　The　experimental　errors　in　thrust　coefficient,　power　coefficient　and　power　loading　calculated　through　＇Kline-McClintock　equation＇　are　less　than　2%.　Additionally,　the　streamline　distribution　and　pressure　of　blade　tip　at　different　rotor　spacings　obtained　from　numerical　simulations　are　presented　to　visualize　the　effects　of　aerodynamic　interference　between　the　top　and　bottom　rotor.　Results　show　that　the　aerodynamic　performance　of　a　co-axial　rotor　with　the　specific　rotor　configure　and　speed　range　can　be　indeed　improved　by　changing　the　rotor　spacing,　and　the　optimal　performance　is　obtained　with　a　rotor　spacing　of　0.19.　Also,　the　magnitude　of　aerodynamic　interference　related　to　the　axial　separation　distance　has　demonstrated　to　be　beneficial　on　the　total　thrust　and　power　consumption.　For　the　same　disc　loading,　a　decrease　in　rotational　speed　results　in　an　increase　in　power　loading　especially　for　z/D　=　0.19.　It　is　also　found　that　the　bottom　rotor　does　affect　the　performance　of　the　top　rotor　at　smaller　rotor　spacings,　whereas　the　effect　is　significantly　reduced　as　the　rotor　spacing　increases.