A method of forming a semiconductor device includes following steps. Firstly, a substrate having a transistor is provided, where the transistor includes a source/drain region. A dielectric layer is formed on the substrate, and a contact plug is formed in the dielectric layer to electrically connect the source/drain region. Next, a mask layer is formed on the dielectric layer, where the mask layer includes a first layer and a second layer stacked thereon. After this a slot-cut pattern is formed on the second layer of the mask layer, and a contact slot pattern is formed on the first layer of the mask layer. Finally, the second layer is removed and a contact opening is formed by using the contact slot pattern on the first layer.