In this work, the effect of dioctadecyl dimethyl ammonium chloride (DDAC, a kind of alkyl ammonium salt) on polar beta phase content and the diameter of electrospun PVDF nanofibers was investigated for the first time. Our experimental results show that the diameter of the electrospun PVDF nanofiber could be largely reduced and the content of polar beta phase also become dominant immediately by just adding a little amount of DDAC. When the mass fraction of DDAC reached 4%, the content of polar beta phase increased by about 39.1% compared with PVDF nanofibers without DDAC. Besides, the crystallinity of PVDF nanofibers also increased with the addition of DDAC. Based on the results, the possible mechanism of cooperative effect between electrospinning and DDAC on fiber diameter and formation of beta phase in PVDF was discussed.