A common mode filter for reducing differential mode signal converting to common mode signal comprising: a core-cylinder, a first coil, and a second coil; wherein two ends of the core-cylinder respectively extend and set with a first flange and a second flange; wherein m1, m2, and m3 turns and n1, n2, and n3 turns are respectively and sequentially wound and set on the first winding portion, the second winding portion, and the third winding portion of the first winding area, the second winding area, and the third winding area of the first coil and the second coil. Therefore, the present invention can effectively reduce the mode conversion characteristic (Scd), and can also improve the noise reduction performance of the product; thereby greatly upgrading the practicality of the present invention; and the present invention has a simple process, therefore the product yield can be further upgraded to achieve the effect.