Surface structure relaxation of organic semiconductors affects the properties of organic devices, although such relaxation has not been well explored. Only two examples have been experimentally reported; tetracene shows a large surface relaxation, while rubrene exhibits no relaxation. Therefore, a systematic investigation of the surface relaxation is conducted on [n]phenacenes (n= 5, 7, and 9). Electron density analyses are performed based on the synchrotron surface X-ray scattering with the aid of first-principles calculations. The results show little surface relaxation in [n]phenacenes.