In this paper, a new compact balanced ultra-wideband (UWB) slot antenna is proposed. The antenna consists of a differential feeding line and a non-uniform multi-band radiating slot. The whole antenna structure is deployed symmetrically across the plane of symmetry (POS) to obtain good differential-mode (DM) radiation characteristic and high common-mode (CM) rejection. In this design, the feeding line is placed across the POS so that in DM operation large current distribution around the intersection can be obtained to result in strong coupling with the slot antenna. On the other hand, the transverse (the direction perpendicular to the POS) dimension of the slot antenna is set as small as possible to push the CM resonant frequencies of the parasitic quarter-wavelength slot far away from the UWB passband so that favorable CM suppression can be obtained. The designed balanced slot UWB antenna shows a DM passband ranging from 3.1 to 10.6 GHz and has a maximum CM return loss of 2.5 dB within the DM passband.