A novel broadband Core-Isolator for modules of multi-standard and multi-band mobile handsets has been developed successfully. A new circuit configuration for extending the isolation frequency band is proposed. The equations of the insertion loss and the isolation bandwidth of a two-port isolator with resonant elements are derived from equivalent circuit using perturbation theory. They show that the isolation bandwidth gets wider depending on the ratio of inductor and capacitor of resonant elements without increasing the insertion loss. A 1.5 × 0.9 × 0.4 mm3 Core-Isolator with two electrodes of asymmetrical structure has been fabricated, which is mounted on a module substrate with resonant elements. It has an insertion loss of 0.5dB and an isolation of 10dB at 824–915MHz which is twice wider than conventional one.