Peroxide-crosslinked EPDMs were examined by dynamic mechanical analysis. In a rubbery state, a plot of storage modulus (E') vs. temperature gave an approximately linear relationship. The slope of the approximated line increased with an increase in apparent crosslinking density (ν_S) which was calculated from equilibrium swelling. However, E' of peroxide-crosslinked EPDMs were not proportional to absolute temperature (T). For the EPDMs crosslinked above 400mol/m³ of ν_S, straight lines approximating the relationships between E' and T gave a constant T-intercept at E'=0. This indicates that E' in a rubbery state is proportional to the difference between T and a reference temperature (T₀). Assuming E' proportional to T-T₀, effective crosslinking density (ν_S) was calculated from E' as T₀=150K. Spinspin relaxation time of network component (T_2n) and the fraction (F_n) were measured by ¹H pulsed NMR with Hahn echo method. At each temperature, 1/T_2n (or F_n/T_2n) showed good linear correlation with E'. Over the temperature range 343 to 423K, the plot of 1/T_2n (or F_n/T_2n) vs. ν_E gave a straight line independent on the temperature at which E' and T_2n were measured.