Several aromatic polyisoimides were prepared, and their third-order nonlinear susceptibilities, χ ( 3 ) (-3ω, ω, ω, ω), were measured by optical third-harmonic generation. Obtained χ ( 3 ) values are in the order of 10 - 1 2 e.s.u. at a fundamental wavelength of 1907 nm. The largest value, χ ( 3 ) =3.4x10 - 1 2 e.s.u., was obtained for the polyisoimide prepared from pyromellitic dianhydride (PMDA) and p-phenylenediamine(PDA). Thermal stability of the polyisoimide prepared from PMDA and PDA was tested. The contents of isoimide structure decreases due to thermal imidization above 200 o C.