A 8-channel wavelength-division-multiplexer (WDM) has been fabricated by using 3-stage cascaded 7 single-mode fiber Mach-Zehnder interferometers. The device not only keeps the merit of super narrow-wavelength spacing but also eliminates the drawback of channel number limit for an individual Mach-Zehnder interferometric WDM. The principles, design considerations and manufacture techniques are described. The experimental sample tested offers 2.4 nm wavelength division spacing and 1.5 dB total insertion loss at 1.5 μm working wavelength.