This study investigates the effectiveness of computer-mediated Engineering Drawing instruction in improving spatial ability namely spatial visualisation and mental rotation. A multi factorial quasi experimental design study was employed involving a cohort of 138, 20 year old average undergraduates. Three interventional treatments were administered, namely Interactive Engineering Drawing Trainer (EDwgT), conventional instruction using printed materials enhanced with digital video clips, and conventional instruction using printed materials only. Using an experimental 3 x 2 x 2 factorial design, the research has found statistical significant improvements in spatial visualisation tasks. There appears to be no improvement in reaction times for Mental Rotation. The authors also have investigated the gender differences and the influence of prior experience of spatial ability. (Contains 2 figures and 4 tables.)