A　compact　low-profile　square　ring　antenna　with　unidirectional　radiation　and　dual　circular　polarization　(CP)　is　proposed　for　uses　in　radio　frequency　identification　(RFID)　readers.　A　small　size　conducting　plane　are　slotted　and　placed　underneath　the　ring　antenna　as　a　reflector　to　achieve　low　profile　and　unidirectional　radiation.　A　branch　line　coupler　is　meandered　inside　the　ring　radiator　to　save　space　and　generate　dual　circular　polarization.　Four　inverted　L-shaped　strips　are　introduced　to　overcome　the　negative　effects　of　tight　coupling　between　the　radiator　and　reflector.　The　measured　-10　dB　impedance　bandwidth　of　vertical　bar　S-11　vertical　bar,　3　dB　axial　ratio　(AR)　bandwidth　and　maximum　gain　are　of　850-1060　MHz,　876-1084　MHz　and　5.62dBi,　respectively.　The　lowest　measured　gain　values　in　RFID　band　of　900-930　MHz　is　4.54dBi　and　the　overall　size　is　0.31　lambda(0)　x　0.31　lambda(0)　x　0.034　lambda(0)　(lambda(0)　is　denoted　the　wavelength　at　center　frequency).　The　proposed　antenna　has　not　only　the　lowest　profile　but　also　the　best　overall　performances.　It　is　an　excellent　antenna　that　can　be　fitted　where　there　is　a　limited　space　in　RFID　system.