We demonstrate a silicon Mach–Zehnder interferometer (MZI) based magneto-optical isolator having an 8 nm bandwidth for more than 20 dB isolation. The operational bandwidth of the isolator is determined by the wavelength dependence of a reciprocal phase difference provided by the asymmetric path length of the MZI. Although a shorter path length requires precise control of the waveguide dimensions, we achieve wider-bandwidth operation.