In this paper, bated on the theory of composited waveguides, the relations between the shape of the crabs-secton of coupling region and polarization sensitivity in wavelength division-multiplexers (WDMs) are analyzed. Then we highlightly compared polarization characteristics of the rectangular cross-section of WDM with those of the elliptical cross-section of WDM, and found that when the ratio of the principal axis to minor axis of ellipse equals 1. 88 the WDMs are insensitive to polarization. The fabricated WDM operating at wavelengths 1480/1550 nm exhibits an isolation above 20 dB, polarization sensitivity of less than 0.1 dB, and excess loss of less than 0.5 dB.