Novel Eu8In17.33S34 (1) is obtained by a solid state reaction and its structure features three parallel slabs of [In9.33S18](infinity), [Eu-8](infinity), and [In8S16](infinity), together with defects in specific lattice sites. It is the first monoclinic ternary M(8-m)In(18-n)Q(34) (M = Pb, Sn, Eu; Q = S, Se) phase member, and the modulation from the ultralong In S bond and lattice site with strong antibonding effect results in the unique structure feature. Its single crystals show anisotropic conductivity and photoconductivity.