The rubber elasticity characteristics of polyurethane network based on hydroxyl terminated polybutadiene(HTPB)-isophorone diisocyanate(IPDI) were investigated. Both sol-gel analyses and solvent swollen compression modulus data were used to evaluate the crosslink density through the variation of NCO/OH equivalent ratio. The density of elastically effective network chains calculated with swollen compression modulus data at 25℃ were 16.788×10^-9 L₀S₀/D²V₀^N (mole/cm³) for gel polymer and 16.788×10^-9 L₀S₀/D² (mole/cm³) for sol-gel polymer, respectively. The polymer-solvent interaction parameters(χ₁) between the present elastomer and solvents like xylene, toluene, chloroform and benzene were evaluated at 25℃. The logarithm of the equilibrium volume fraction (V_2e'') in gel polymer, which was in accordance with the prediction of the Flory-Noggins schema. The effective network chain lengths (M_e) of these elastomers at NCO/OH equivalent ratio=0.95-1.10 were 8,000-12,000(gr/mole).