A novel method is proposed to improve the sensitivity of DOP(degree of polarization) ellipsoid on the polarization mode dispersion(PMD) in 40 Gb/ s non-return-to-zero(NRZ) systems by using optical bandstop filters. The effects of filter′s bandwidth on the three axes of DOP ellipsoid are investigated numerically. The results show that the sensitivity of DOP ellipsoid on differential group delay(DGD) and principal state of polarization(PSP) rotation rate is greatly improved by using a filter. Considering the loss induced by the filter, the bandwidth of the filter should be limited under 10 GHz when the maximum DGD is equal to 20 ps. Additionally, the results also show that the polarization dependent chromatic dispersion(PCD) which is smaller than 40 ps2 has less effects on the three axes of DOP ellipsoid when the bandwidth of a filter is equal to 10 GHz.