A helical nanostructure can be obtained by self-assembly of a diarylethene derivative that bears two malononitrile substitutes in a tetrahydrofuran/water medium. It is revealed that the helical nanostructure changed from helical nanofiber to helical nanotube when the diarylethene monomer changed from the open-ring isomer to the closed-ring isomer upon irradiation with 365 nm ultraviolet light, meanwhile, the helix angle of the nanostructure changed from 50 degrees +/- 5 degrees to 75 degrees +/- 5 degrees. There is a great possibility that the helical nanofibers and helical nanotubes are assembled from dimers as base units based on theoretical calculation and experimental results.