A gravity sensor has a photo interrupter and a gravity wheel. The photo interrupter has a housing, a cavity in the housing, a light emitter on one side of the housing and a light detector on the other side of the housing. The gravity wheel points in a direction of gravity and has a pivot at center thereof and pivotally arranged on the housing, a plumb at one edge of the gravity wheel and a plurality of through holes annularly arranged in the gravity wheel. The gravity wheel has a portion present in the cavity. An inclined angle of a test object is determined by calculating relative movement between the photo interrupter and the gravity wheel.