The optical-limiting property of a new cyclic phenylacetylene oligomer was investigated. Toluene solutions of the oligomer contained within a quartz cell were irradiated with the pulse from frequency doubled Nd : Yag laser at 532 nm. At low input fluence, the transmittance of the toluene solution was constant and agreed well with that obtained by spectrophotometer. At high input fluence, the transmittance of the solution above a concentration of 5 wt % decreased with input fluence. The oligomer had an optical limiting property. As the concentration of the oligomer increased from 5 to 20 wt %, the threshold for optical limiting decreased. The threshold was independent of the molecular weight. The optical limiting property was analyzed by the following equation obtained according to the reverse saturable absorption mechanism; log(I-0/I) = K(I-0 - I) + A(g), where I-0 is the input fluence, I is the output fluence, K is a parameter depending on the absorption cross section and the relaxation time, and A(g) is the absorbance of the ground state.