This paper presents a balanced small antenna matched with the RF chip CC2500. The antenna is confined in an area of 3mm∗11mm and the whole RFID system is printed on a microSD substrate. The produced microSD card is inserted into a mobile phone as a plug-in. The geometry of the antenna is optimized to obtain a specific radiation pattern, which ensures that the main power can be radiated out through the slot. Then we introduce a measurement method of the input impedance of a balanced antenna in experiments and the results show the error is below 5% under the restrictions.