Abstract: Curing reaction of the matrix resin (MR) developed by the authors of this paper was studied for the composites structural repair application through in-situ FTIR. Conversions of the epoxy radical at different curing temperatures were calculated and were compared with the gelation time of the MR system. Conversions of the epoxy radical were also compared with the viscosity changes vs. time at the same temperatures. When the curing temperature reaches the onset curing temperature (108℃) on DSC trace of MR, the conversion curve from in-situ FTIR tallied with the gelation time and the curve of viscosity vs. time by means of macroscopical methods. When the curing temperature is below 108℃, the conversion curve from in-situ FTIR will deviate from the curing degrees concluded from the gelation time and the curve of viscosity vs. time at the corresponding temperature. The main causes of the deviation were analyzed.