Analysis of the gear system as rotors is a very challenging problem because the system is a composite rotor; therefore possesses very complicated dynamic behavior. For example, the whirling motion and critical speed of the system has to be found by considering all the rotors in the system as well as interactions between them, which makes the solution process as well as proper interpretation of the solution extremely complicated. Complex variable approach, which is proposed for the analysis of a single rotor system, is very powerful for this purpose. The approach is expanded to the analysis of combined rotor system to apply it to the gear system analysis.