Internal friction as a function of strain amplitude in zone-refined molybdenum single crystals and commercially pure polycrystalline molybdenum was measured with the three-component piezo-electric resonator method developed by Marx. A Granato-Lücke (GL) type of strain-amplitude-dependent decrement was obtained in molybdenum single crystal annealed in hydrogen at 2000℃. With some prior deformation in molybdenum single crystal, a plateau (or maximum) appears on the decrement-amplitude curve, which is similar to the results of pure aluminium at low temperature and low strain amplitude. From the observations of dislocation etch-pits, it may be concluded that the presence of this plateau is related to the fresh dislocations (or weakly pinned dislocations) produced by cold work. The increase of decrement at the plateau (or maximum) is proportional to the density of fresh dislocations. The investigations of the effects of temperature and aging (200-300℃) on the amplitude-dependent decrement indicated that the decrement decreases with the decrease of temperature and with the increase of aging time, and the position of the maximum shifts to the higher amplitude as the temperature decreases. All the results are discussed in terms of an extension of the GL theory.The effect of cold work on the strain-amplitude-dependent decrement of poly-cryctalline molybdenum was also measured; the results are fairly consistent with the GL theory.