A compact microstrip low-pass filter (LPF) with ultra-wide stopband using the analysed triangular-shaped resonator is presented. To achieve LC equivalent circuit of the triangular-shaped resonator, the structure of a rectangular-shaped resonator is adopted. To investigate the behaviour of the proposed triangular-shaped resonator, transfer function of the main triangular resonator is calculated. The designed LPF has −3 dB cut-off frequency of 1.52 GHz with the stopband bandwidth around 12fc and total circuit size of 0.062λg × 0.104λg . The proposed LPF has the figure of merit of 46,718, which shows its good performance. The proposed filter is designed, simulated and fabricated. It is observed that the measurement results are well matched with simulation results.