A bulk acoustic wave resonator including a substrate; an air cavity formed on the substrate; and a resonating part formed on the air cavity and comprising a first electrode, a piezoelectric layer, and a second electrode which are sequentially laminated, wherein a cross section of the air cavity has a short side, a long side opposing the short side, a first lateral side and a second lateral side connecting the short side and the long side to each other, the first and second lateral sides are inclined, and a surface roughness of the first electrode, the piezoelectric layer, the second electrode, or any combination thereof is between 1 nm and 100 nm.