Twin boundaries (TBs) in ultra-fine grained (UFG) copper prepared by powder metallurgy are investigated using high-resolution transmission electron microscopy (HRTEM) and Geometric Phase Analysis (GPA). Specimens before and after mechanical deformation (compression of 40%) are analysed and emphasis has been made on study of TBs defects. Twin boundaries in the as-processed specimens are in majority disoriented from the perfect S3 coincidence. They present a faceted structure with coherent {111} and incoherent {112} facets. The latter have a 9R structure and the {111}/{112} junctions are associated with sessile dislocations of Frank type (b=a/3<111>). Shockley glissile dislocations with Burgers vector of type b=a/6<112> are also present. This microstructure is interpreted in terms of the absorption and decomposition at room temperature of lattice dislocations (60° type). After mechanical deformation, an enrichment of twins in dislocations and a decrease of step density and height is observed and quantified by statistical analysis. Deformation mechanisms of UFG copper are discussed in the light of these observations.