Degenerate four-wave mixing method was employed to determine second molecular hyperpolarizabilities γ of a series of thiahetero [N] helicenes (N=3, 5, 7) and their methyl derivatives. γ values were calculated from the measured bulk susceptibility χ ⁽³⁾ of benzene solution. It was found that γ increases by about one order of magnitude with the addition of onebenzothiophene unit. The largest value of γ was about hundred times larger than that ofcarbon disulfide. The comparison of the optical absorption spectra measured before and afterlaser irradiation indicated that two chemical processes induced by photon absorption wereinvolved.