In this paper, a planar sub-wavelength resonator constructed by interdigital capacitors and meander microstrip line is presented. Under differential-mode (DM) excitation, the planar sub-wavelength resonator is virtually connected to ground, it is equivalent to composite right/left-handed transmission-Line (CRLH-TL) resonator. Under common-mode (CM) excitation, the common modes (CMs) are far away from the DMs, which can introduce good CM suppression. Based on this resonator, a balanced dual-Band bandpass filter (BPF) with ultra-compact size and perfect CM suppression is proposed, two center frequencies are located at 2.4 GHz and 3.6 GHz, respectively. The simulated and measured results show perfect DM response and high CM suppression. This balanced dual-band filter can be used in microwave -RFID and 5G application.