A method for predicting the resistance to heat deterioration of a sulfur-vulcanized isoprene rubber is disclosed. The nuclear magnetic resonance spectrum of the isoprene rubber is obtained by the use of a solid state nuclear magnetic resonance method employing magic angle spinning. The spectrum of a cross-linked structure α and the spectrum of a cross-linked structure β in the nuclear magnetic resonance spectrum are identified. The percentage of the cross-linked structure α and the percentage of the cross-linked structure β in the overall cross-linked structures of the sulfur are computed from the spectrum. From the computed percentages, the resistance to heat deterioration of the isoprene rubber is predicted.