The plastic deformation of semi-crystalline polybutene (PB) has been studied at the micrometric and nanometric scales by Atomic Force Microscopy (AFM). Owing to a movable tensile drawing stage, capturing images from the same locus of the sample allowed for quasi in situ observations of the plastic processes. In the case of PB films having an average spherulite diameter of about 20 μm, the macroscopic deformation was homogeneous over the whole gauge length of the sample, up to rupture. In parallel, the local deformation at the scale of the spherulites was very close to homogeneous and obeyed an affine deformation law over the whole strain range: the shape of the deformed spherulites was kept roughly elliptical up to rupture without clues of fibrillar transformation. The inter-spherulitic boundaries displayed very high cohesion. Fragmentation of the crystalline lamellae proved to be a predominant process, while crystal slip could not be detected at the scale of the AFM resolution, i.e. a few nanometers. Wide-angle and small-angle X-ray scattering yet revealed the occurrence of crystal plastic shear. Similar observations have been made in the case of PB films having an average spherulite diameter of about 5 μm. In the conclusion, a comparison is made with a previous study regarding the deformation mechanisms of a PB sample having 200 μm wide spherulites which displayed brittle behavior.