The structures and properties of highly excited Rydberg and mixed Rydberg-valence states of molecular chlorine have been studied in detail by combining ioniza-tion, excitation and fluorescence spectroscopic techniques following multiphoton exci tation. The position of the inner-well of 11∑u+ state is located, and the 21∑u+ state is found to exhibit a strong mixed Rydberg and ion-pair valence character. A new series with symmetry is discovered and a assignment to the excitation of Rydberg state 3∑u is suggested. Based on the experimental results, the processes of three photon excitation, interation of 21∑u+ with 11∑u+ states and spectral effects of third harmonic generation are discussed.