Separation of periodic signals is a fundamental problem in signal processing. If two signals are mixed and overlap in time domain and in frequency domain, the conventional filtering techniques appear to be inefficient. In this paper we present an algebraic theory which may be more suitable to the signal separation problem. Conceptually, the new theory is simple and different from the filtering theory, and it may be usable for separation of quasi-periodic signals. Numerical examples are presented to show the significance of the theory.