Miniaturized periodic stepped-impedance ring filter with patterned ground plane is proposed. The size of a ring filter can be significantly reduced, along with an extended upper stopband by introducing the periodic stepped-impedance structure and patterned ground plane. Based on the even- and odd-mode networks, the mode splitting characteristics and impact of the patterned ground plane on the electrical performances of the filter are investigated. Experimental results validate the theoretical analysis and design procedures with good agreement.