Thermal behaviors of complex refractive index of merocyanine (MC) Langmuir-Blodgett (LB) films have been investigated with absorption spectrum measurements, which are obtained from the Kramers-Kronig (K-K) analysis. The complex-refractive-index spectra with an anomalous dispersion around a J-aggregate absorption band of MC molecules are changed by annealing temperatures. This thermal characteristic is attributed to the annealing-changeable MC J-aggregates. The MC J-aggregates in the LB films collapse in the annealing and become a monomer state. The refractive-index dispersion decreased with the collapse of MC J-aggregates. These behaviors are confirmed from the reflection spectrum measurements under the different conditions of incident light to the MC film samples.