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Recently it was reported that a periodic nanostructure was formed by femtosecond pulse laser (λ = 800 nm, fr = 1 kHz) irradiation at the fluence slightly above the ablation threshold over the surface of diamond-like carbon (DLC) films. Our previous studies have demonstrated that the nanostructure can decrease a friction of the surface. However, because DLC films have the various features according to the binding conditions, it is important to clarify a formation mechanism of the nanostructure for controlling the shape. In this paper, we reveal a relationship between laser irradiation conditions and the shape of nanostructure such as pitch and depth experimentally. Furthermore we adapt theoretical curve on interaction between laser beam and electron plasma wave, and the experimental results. As a result, it suggested that the formation of nanostructure with low fluence is caused by interaction between laser beam and electron plasma wave, and the pitch of nanostructure is finer with less ablation than conventional mechanism.