Silicon-on-Insulator (SOI) wavelength demultiplexers are studies by using thelarge cross-section single-mode rib waveguides conditions and the two-mode interferenceprinciple in silicon direct bonding SOI with bsck-polishing.They are fabricated by usingKOH anistropic etching. Their insertion loss are measured to be down to 4. 81 dB at thewavelength of 1.3 μm, and crosstalk is less than -18.6 dB at 1.3 μm.