This paper presents a theoretical consideration on the possibility of subsynchronous resonance (SSR) in longitudinal power systems. Shunt capacitors are used for reactive power compensation in our country, but series capacitors are not used in general. The possibility of SSR is therefore small. However, if power transmission increases, and accordingly, if shunt compensation increases in amount, there is no guarantee that SSR will never occur. First, we investigate network impedance viewed from a generator. Its resonance frequencies become lower with increasing transmission power. One of them gets subsynchronous if the power exceeds a certain value. In this area, there is some possibility of SSR, which is confirmed with the damping property of the generator. The admittance matrix of the load buses is singular at the resonance frequencies. Their number is equal to the dimension of the matrix. The frequencies are common to all generators but not limited to one particular generator. One of them becomes equal to 60 Hz as we increase transmission power. We regard this power as a limit for SSR. However, steady-state stability limit is lower than this limit, and steady operation is not possible at the limit. Therefore, it is impossible to enter the area of SSR. Thus, we conclude that SSR does not occur in shunt compensated systems. However, this property is easily lost if some series compensation is introduced.