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An anomalously big jump of thermal expansion in the c direction of the Bi1.7Pb0.3Sr2Ca2Cu3O10+y has been observed at a temperature slightly above the superconductivity transition temperature, Tc. The jump is one order of magnitude bigger than the estimated value with the classical theory of superconductivity. This anomalously big jump vanishes after the samples were annealed in O2 for a long time and became nonsuperconducting. The correlation between the anomaly and the Tc support the view that electron–phonon interaction as well as superconductivity is enhanced by anharmonicity of phonons in high-Tc cuprates.