A novel center-fed wideband slotline antenna is proposed using the multi-mode resonance concept. By symmetrically introducing one or two pairs of slot stubs along the slotline resonator near the nulls of electric field distribution of the second odd-order mode, two modes are excited in a single slotline radiator. With the help of these stubs, the second odd-order mode can be gradually merged with its first counterpart, resulting in a wideband radiation characteristic with two resonances. Prototype antennas are then designed and fabricated to experimentally validate the principle and design approach. It is shown that the operation fractional bandwidth of the proposed slotline antenna could be effectively increased to 31.5% while keeping an inherent narrow slot structure.