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In this paper initially a parallel coupled bandpass filter with centre frequency 2.4 GHz and fractional bandwidth 20%, has been designed and simulated. Hence, the out-band rejection performance of this parallel coupled filter has been improved by using a proposed “Coupled Closed Loop Square Resonator Structure”. Using this proposed structure, an additional transmission zero or attenuation pole is introduced in the harmonic passband region extending from 4.4 to 5.1 GHz improving the out-band rejection of the filter. The filter is designed on FR4 substrate having relative permittivity 4.4 and substrate height 1.59 mm using IE3D full wave electromagnetic simulator.