Much research has been conducted to investigate the quality of writing and high-level revisions in word processing-assisted and pen-and-paper writing modes. Studies that address cognitive aspects, such as experience and comfort with computers, by which students compose essays during writing assessments have remained relatively unexplored. To fill this gap, the present study investigates the impact of writing using computers versus traditional writing with pen-and-paper based on 366 undergraduate students in Singapore. It analyses the students' thinking processes, the quality of their written work, and their perceptions of working with computers. The results indicate that the students demonstrated a positive attitude towards working with computers. Furthermore, the computer-delivered writing mode has a positive impact on their quality of writing in both technical aspects (content, organization, vocabulary, language use, and mechanics) and global aspects (setting macro rhetorical goal and ordering information in achieving the macro rhetorical goal). On the other hand, the results show that there was no significant difference between the paper and computer writing modes in their effects during the planning stage of writing in-class reflections. The study contributes to writing research by encouraging teachers to give students an option of delivery modes, beyond traditional pen-and-paper, for their writing assessments.