Sigma = 9 [0 (1) over bar 1]/(1 2 2) symmetric tilt grain boundary in Cu is investigated using high resolution transmission electron microscopy (HRTEM). Modified mirror and glide-mirror symmetric structural units (SUs) are found to coexist in the grain boundary, Partial dislocations with Burgers vectors close to half a displacement shift complete (DSC) lattice are introduced to connect the glide-mirror symmetric structural unit (SU) with the modified mirror-symmetric one in the grain boundary. The Burgers vectors are analyzed to be close to a/18 [1 2 2](L) // a/18 [(122) over bar](R) and a/36 [(4) over bar 1 1](L) // a/36 [(4) over bar 1 1](R). The volume expansion per unit area of the grain boundary, deltaV/A, in the glide-mirror symmetric region is determined as -0.00238 nm which is a little smaller than that in rigid-body model, while the deltaV/A in the modified mirror symmetric region is computed as 0.03221 nm which is larger than that in the same model. The two values of deltaV/A are corresponding to compressive and tensile stress field, respectively. One possible mirror and glide-mirror symmetric structural model is proposed according to atomic relaxations. (C) 2002 Elsevier Science B.V. All rights reserved.