Methods related to the design of gain flattening filters for amplified spontaneous emission (ASE) and erbium-doped fiber amplifier (EDFA) are reviewed. A novel scheme to realize the gain flattening filter by using a piece of twin-core fiber and a wavelength division multiplexing (WDM) coupler is proposed and verified experimentally. The spectrum of the twin-core fiber can be adjusted by bending, twisting and stretching to meet the filter requirement. The flexibility and tunable properties make the proposed all-fiber device a good candidate for gain flattening filter. The experimental results show that the ASE spectrum can be flattened in the wavelength region of 1527.36～1561.04 nm with ripples within ±0.5 dB. The results show the filters with twin-core fiber may offer great potential for optical communications and sensing applications.