A new type of lowpass filter with an ultra-wide stopband, sharp transition and compact size is proposed. To reduce the circuit size, a bended transmission line is adopted in the stepped impedance resonators. To achieve a sharp roll-off, a pendulum-shaped resonator is implemented in the filter. The transition band is from 1.30 to 1.45 GHz with -3 and -20 dB, respectively. The stopband with an attenuation level better than 20 dB is from 1.45 up to 23.4 GHz, hence an ultra-wide stopband with 18th-harmonic suppression is achieved. Furthermore, the proposed filter exhibits an extremely small size of 0.09λg × 0.11λg where λg is the guided wavelength at the cutoff frequency.