Aggregation behavior of three kinds of benzindocarbocyanine dyes in aqueous methanol solution was studied by UV-Vis absorption spectrum. The results indicated that the three dyes all existed monomer-dimer equilibrium in aqueous methanol solution (concentration range $10^{?5};to;10^{?6}$ M) at 25.0$sim$41.0 ${^{circ}C}$ for Dye 1, 28.0$sim$49.0 ${^{circ}C}$ for Dye 2 and 26.0$sim$47.0 ${^{circ}C}$ for Dye 3. The fundamental property of the three dyes as the dimeric association constant KD, the dimeric free energy ${Delta$}G_D, the dimeric entropy ${Delta$}S_D</TEX>, and the dimeric enthalpy ${Delta$}H_D were determined. The ${Delta$}H_D</TEX> of three dyes: Dye 1, Dye 2 and Dye 3 was -42.5, -15.1 and -18.9 kJ/mol, respectively. The experimental observations were the subject of a theoretical study including the ground-state geometries which were fully optimized using DFT at B3LYP/6-31G level. The effect of dye molecule structure on ${Delta$}H_D was discussed by theoretical calculations.