The　electronic　structures　and　optical　properties　of　4-N,　N-dimethylamino-4＇-N＇-methylstilbazolium　tosylate　(DAST)　and　4-N,　N-dimethylamino-4＇-N＇-methylstilbazolium　2,4,6-trimethylbenzenesulfonate　(DSTMS)　were　investigated　using　density　functional　theory　based　on　the　plane　wave　basis　set.　The　results　indicated　that　the　two　compounds　showed　similar　band　structures,　and　the　top　of　the　valence　band　and　the　bottom　of　the　conductive　band　mainly　originated　from　the　N　2p　states　of　dimethylamino　and　methylpyridine,　respectively.　In　terms　of　the　linear　optical　properties,　the　birefringence　indexes,　Delta　n,　of　the　two　compounds　were　very　large　(Delta　n＞0.5),　and　they　exhibited　good　light　transmission　in　the　mid-and　far-infrared　regions.　With　regard　to　second-order　nonlinear　optical　characteristics,　the　DAST　and　DSTMS　crystals　showed　strong　second　harmonic　generation　(SHG)　responses,　and　the　corresponding　SHG　coefficients　(d(11))　were　about　150　pm.V-1.　Analysis　of　the　band　structures　showed　that　the　SHG　responses　of　the　two　compounds　were　closely　related　to　charge　transfers　between　electron-donating　and　electron-withdrawing　groups.　Ethylene　bridging　also　played　an　important　role　in　the　charge　transfer　process.