On the base of the composited waveguide theory, the characteristics of coupled optical power and poarization in the coupling region of a overlong coupler have been analyzed, and a new mothed to fabricate single mode optical fiber dual window narrowband wavelength division multiplexers(NWDM) is proposed and demonstrated experimentlly. Three fabricated devices, operating at 1300 nm, 1550 nm mono window, and 1300 nm/1550 nm dual window, respectively, show spacing channel of 14 nm, channel bandwidth of 2～3 nm, adjacent channel isolation 20 dB, <0.1 dB polrization sensitivity, and <0.5 dB optical loss. These specifications are the best performances reported to-date for the NWDMs.