Palladium coated niobium foil (Pd-Nb-Pd) is widely used as the hydrogen and deuterium-permeable materials for both the hydrogen purification and the hydrogen pumping. However, the performance of Pd-Nb-Pd membranes is usually affected by the degassing treatment (DT) that is typically implemented during the permeation test. In this paper, effects of DT on the deuterium permeability of the Pd-Nb-Pd composite membranes are investigated with varying degassing time. The deuterium permeability test is performed in the temperature range of 573-723 K at the driving pressures of 1-5kPa. Results show that the permeation behavior of Pd-Nb-Pd membranes is affected by DT at 673 K during deuterium permeation. With the increasing DT time, the permeation performance deceases. The deuterium permeability of the sample heated at 723 K is 1.23 x 10(-7) mol m(-1).s(-1).Pa-0.5 at the 10 min DT time, one order of magnitude higher than 5.33 x 10(-8) mol m(-1).s-1.Pa-0.5 of the sample at the 60 min DT time. The corresponding apparent activation energies are 20.11 kJ/mol and 52.55 kJ/mol for the samples at 10 min and 60 min DT time, respectively. Scanning electron microscope and Auger electron spectroscopy analysis reveal that a large number of pores are formed in the top Pd coating layer, and the inter-diffusion at the Pd-Nb interface occurs during the permeation experiment. Therefore, it is suggested that both the surface microstructure change and the inter-diffusion are attributed to the decline of deuterium permeability.