Cyclic nanoindentation tests were carried out on Zr50Cu40Al10 bulk metallic glass (BMG). From the designed quasi-static and incremental cyclic experiments at loading rates ranging from 250 to 2500 mu N/s and at ambient temperature, we observed significant difference in inelastic deformation as a function of solicitation modes. Under incremental cyclic loadings, the hardness of the BMG increased each time when the sample was reloaded immediately after unloading and then gradually reduced before the next unloading. In contrast, we found that quasi-static cyclic loadings induced a mechanical softening which appears to be dependent on the number of cycles and the loading rates. The inelastic deformation was studied by analysing the remnant indent morphology using atomic force microscopy. A free-volume mechanism was proposed for interpreting these observations quantitatively.