This report presents the synthesis, growth, crystal structure determination and second-harmonic generation efficiency of nonlinear optical co-crystal 4-N, N-dimethylamino-4-N-methyl-stilbazolium (m-nitrobenzenesulfonate)0.6I0.4 (DSIMNS). It is co-crystal of two DAST (4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate) derivatives. X-ray crystallographic analyses revealed that the crystal structure of DSIMNS is monoclinic, with space group P21. Kurtz powder test has shown that DSIMNS exhibits a large second-harmonic generation efficiency 500 times of that of urea.