Herein an on-chip area-saving balun is presented for a multiband transmitter that supports all the commonly used standards (NB-IoT, BLE and ZigBee) in low-rate IoT. By switching the effective number of coil turns in this balun, it can guarantee a Q value higher than 10 and provide the required inductance to supporting three-bands (LB: 824-915 MHz, MB: 1695-1710 MHz, HB: 2400-2483 MHz). Compared with the traditional multi-transformer design method in multiband transmitter, the tunable balun can effectively save the chip area. The proposed balun was designed in a 65-nm RF CMOS process, its total area is only 0.14 mm 2 . Simulation results show that its insertion loss is less than -3.1dB, the amplitude and phase balance are both with in 1.2dB and 1°.