The influence of ligand length on helix-coil transition parameters in the presence of different ligand concentrations has been considered theoretically. A decrease of this influence with an increase of ligand length when the binding constant has a constant value was shown. When binding free energy is proportional to the ligand length, i. e. when K = K0m (K-binding constant, m-ligand length) the length effect is unambigous. In the presence of low-ligand concentration the stability of DNA double helix increases with their length, whereas in the presence of high concentrations it decreases.