Embodiments of mechanisms for forming a semiconductor device structure are provided. The semiconductor device structure includes a semiconductor substrate. The semiconductor device structure further includes a dielectric layer on the semiconductor substrate. The semiconductor device structure also includes at least one conductive structure embedded in the dielectric layer. A plurality of crystal grains are composed of the conductive structure, and a ratio of an average grain size of the crystal grains to a width of the conductive structure ranges from about 0.75 to about 40.