We investigate the longitudinal static second hyperpolarizabilities (γ) of open-shell singlet one-dimensional systems. The intersite interaction effects on the average diradical character and size dependences of γ are examined using hydrogen chain models (H2n) with different chain lengths and bond-length alternations (BLA) by the ab initio MO full-CI and coupled cluster methods. These systems exhibit significant enhancement of γ/n in the small average diradical character region compared to diradical H2. The BLA is found to reduce the maximum γ/n and to move the diradical character giving the maximum γ/n to intermediate region. These systems also show remarkable chain-length dependences of γ/n (comparable to those of closed-shell systems) in the small average diradical character region.