Thermal instability of GaAs-AlAs and GaAs-InAs superlattices was studied by Raman spectroscopy. Samples with various periods and grown at various temperatures were investigated to understand the stability of interface in superlattice and to obtain an activation energy of mixing by thermal annealing. Mixing occurred more easily in GaAs-InAs strained superlattice than GaAs-AlAs strain-free superlattice. Samples grown at a lower temperature are more stable against thermal annealing than those grown at a higher temperature.