Abstract: The ageing effect of the supercondu ctivity of La_0.67 Ca_0.33 MnO₃/YBa₂ Cu₄ O₈/La_0.67 Ca_0.33 MnO₃ films was studied.After 11-month in air,compared with the as-deposited samples,all of the samples get lower superconducting transiti on temperature T_C and all the series show larger critical thickness d^cr_Y below which the T_C disappear.Accelerated ageing tests of La_0.67 Ca_0.33 MnO₃-coated YBa₂ Cu₄ O₈ films show that thin La_0.67 Ca_0.33 MnO₃ film can efficiently protect the YBa₂ Cu₄ O₈ films from water attack when its thickness d_L > 40nm.