In this work, MNA (2-methyl-4-nitroaniline) and substituted carbazoles were incorporated into silica matrix to for composite thin films on the glass substrate by a sol-gel process for NLO applications. In the composite thin films, the thermal stability was dependent on the type of functional groups incorporated into the organic compounds. DR 1(disperse red 1) and carbazole-2 (5-nitro-9-hydroxyethyl-cabazole) which has a terminal -OH group showed enhanced thermal stability, whereas carbazole-1 (5-nitro-9-ethyl-carbazole) and MNA, which don’t have a reactive functional group, showed rapid degradation upon thermal treatment. This strongthly suggests that the thermal stability of incorporated organic compounds can be enhanced by a reactive terminal group in organic compounds without severely damaging NLO properly.