The Σ=9(122) <011> pure tilt silicon bicrystal has been deformed. The evolution of its structure under stress has been studied in details. It was shown /1,2,3,4,5/ that the deformation induced dislocations generally enter the grain boundary (GB) where they decompose into elementary GB dislocations (GBD'). GBD's react by glide and climb. The GB migrates when GBD's associated with step are moving along the interface. This leads to appreciable changes of the original GB structure whereas the misorientation angle between the two grains varies. This paper deals with the structure determination of all the defects involved in the previous transformation and of the different GB's structures encountered during the process. The HREM observations were performed on the JEOL 200CX (Cs=1.05mm) to avoid electron irradiation damage in silicon and the images were simulated using a dedicated computer.i) Initial Σ=9 (122) GB (θ = 38.94°): the initial structure has been studied /6,7,8/. The main feature is the zig-zag stacking of the 7-5 atom rings (called M structural unit Jin the (122) common plane.In the following the stacking of the GB period will be described by the sequence M+M− (M+ is the mirror of M− with respect to the GB plane).