In the organic quasi-one-dimensional conductor TTF-TCNQ, the successive structural phase transitions have been studied in relation to the ordering of the charge-density waves (CDWs), which are the coupled modes of lattice distortions and one-dimensional electrons. The origin of the successive occurrence of the transitions is in the relative independence of the CDWs on each kind of molecular stack from those on the other. In fact, above the metal-insulator transition temperature 53K, the CDW on each stack is independent and has only one-dimensional correlations, and its correlation length is short above 53K , In a system with two relatively independent kinds of CDWs, independent ordering of each kind of CDW at a different temperature may be possible. Experimentally and theoretically, there have been identified three transition temperatures, T1 (53K), T2 (49K) and T3 (38K) in TTF-TCNQ [2,3]. T1 is characterized as the three-dimensional long-range ordering temperature of the so-called 2kF-CDWs on the TCNQ stacks. At T2, the long-range order of the TTF-CDWs begins to develop, although it is considered that this is an ordering induced by the ordered TCNQ-CDWs. Once two kinds of CDWs have long-range order, the coupling between them must be taken into account, since the magnitude of both order parameters becomes appreciable. Thus, the interactions among TCNQ- and TTF-CDWs brings about the decrease of the transverse wave number Qa from 0.5a* with decreasing temperature  due to the competition of TCNQ-TCNQ and TCNQ-TTF interactions.