All-optical signal processing is a key technology in high speed optical communication network. The label/head processing as a key technology of optic packet smitching can be realized by using the all optical comparator. A 3-bit all optical comparator based on cascaded semiconductor optical amplifier-Mach-Zehnder interferometer (SOA-MZI) is demonstrated at 10 Gb/s by simulation. An extinction ratio of 48 dB and peak power values higher than 7 mW are found at the output. A fall width at half maximum (FWHM) of 8.5 ps is obtained for the output pulses from the comparator. The output pulses have a high intensity and a high stability.