A piezoelectric actuator and a method of measuring a motion by using the piezoelectric actuator are provided. The piezoelectric actuator includes: a movable member that is disposed to face the fixed member; a piezoelectric element that is disposed between the fixed member and the movable member, and configured to operate in a shear mode based on input voltages applied to the piezoelectric element and move the movable member relative to the fixed member; and a position sensor that is disposed between the piezoelectric element and the movable member, and configured to measure a motion of the movable member.