The collapses of granular materials frequently occur in nature in the form of, for example, rock avalanches, debrisavalanches and debris flow. In previous studies of collapses of a granular material, most of the focus has been onthe effect of initial geometry and mechanical properties of the granular materials, the run-out distance, and thetopography of final deposit. In this study, results of analogue models and scanned natural failed slopes are usedto outline the mode of failure of an unstable slope. Model results and field observations are used to argue that agranular mass moves downslope in a wavy pattern resulting in its intensive deformation.In the models, we mainly investigated the internal deformation of collapses of granular slopes in terms of theirinternal structures and the spatial and temporal distribution of the latter. Model results showed that a displacedmass of the granular slope has the following two features: (1) Initial collapse resulted in a series of normal faults,where hanging-wall blocks were slightly deformed, like the slump-shear structures in nature; (2) With furthercollapse, a set of secondary structures, such as deformed/folded fault surfaces, faulted folds, displaced inclinedfolds, and overturned folds formed near the slope surface. The occurrence of these structures reflects the failureprocess of the granular mass in space and time. In addition, our model results show that the nature of basal frictionhas a significant influence on the geometry and kinematics of these structures at the slope toe. Model results showalso that the mass does not glide downslope along only one surface, but includes several gliding surfaces each ofwhich take part of the sliding. These gliding surfaces become steeper deeper in the sliding mass. Some of thesefeatures observed in the models are also detected in the field. Scanned failed slope surfaces show a wavy patternsimilar to that in the models, reflecting the presence of normal faults at the head of the slope and folding at theslope toe.