This paper investigates the internal stability of one-degree-of-freedom (l-DOF) and two-degree-of-freedom (2-DOF) linear time-invariant structural vibration systems. Five types of vibration systems are involved - systems with both boundaries fixed, systems with one boundary fixed, systems with no fixed boundary but the displacement of one boundary is restricted, systems with one boundary fixed and the other restricted, and systems with both boundaries restricted. The results give the necessary and sufficient conditions for the systems to be internally stable and can provide great convenience when one evaluates the internal stability of a vibration system.