In this paper, an isosceles triangular patch resonator with an inverted V-shaped slot pattern is fabricated to realize a miniaturized microstrip dual-mode filter. The proposed structure possesses the elliptic characteristic and has the advantage of size and loss reduction. After analyzing the results, the relationships between the filter characteristics and the geometrical parameters of the inverted V-shaped slot are derived. The dual-mode filter exhibits a fractional bandwidth of 14.5% centered at 1.93 GHz with the lowest insertion loss of 1.4 dB. It is shown that a size reduction of 37% is achieved compared with a conventional dual-mode triangular filter. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1969–1974, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21824