This study explored graduate teaching assistant (GTA) grading on 480 papers across two writing assignments as integrated into large Introductory Psychology courses. We measured GTA accuracy, consistency, and commenting (feedback) quality. Results indicate that GTA graders improved, although unevenly, in accuracy and consistency from Time 1 to 2 when compared to professional graders who scored the same papers. GTA commenting on papers improved from Time 1 to Time 2 as well, as indicated by seven measures of feedback effectiveness. However, subsequent to training GTAs gave higher grades than professional graders, and significant interactions between GTAs and time were evident in terms of both grading and commenting improvement. Conclusions for GTA training and professional development are discussed.