The nonlinear optical property of RbCdI 3 .H 2 O (rubidium cadmium iodide monohydrate, RCIM) has been identified. The optical second harmonic generation effect has been measured by using the Kurtz powder technique to be 3.6 times as large as that of KDP (KH 2 PO 4 ). The infrared spectrum indicates RCIM is transparent in most of the infrared region. The calculation shows the band gap of the crystal is 4.1 eV, which is larger than that of LiNbO 3 . Single crystals sized up to 20x4x2 mm 3 have been grown in the aqueous solution by a slow evaporation technique.