A substrate processing apparatus may include a substrate support unit in a lower portion of a process chamber; an edge ring on and along an edge of an upper surface of the substrate support unit and protruding from the upper surface of the substrate support unit; and a substrate alignment sensor on the substrate support unit. The substrate alignment sensor may include a body, an imaging unit, and a control unit. The imaging unit may be on a bottom surface of the body and configured to image a sidewall of the edge ring and a sidewall of the body. The control unit may be configured to control the imaging unit and calculate a spacing between the sidewall of the edge ring and the sidewall of the body. The control unit may be configured to align the substrate alignment sensor with the substrate support unit based on the spacing.