The aim of this study is to investigate the effect of digital video screen size on cognitive achievement among High and Low Perceptual Speed Students', The study sample consists of ninety nine 99 students aged between 21-23 years old from fourth year students enrolled in the department of instructional technology, faculty of education benha university 2010-2011 in the second term, the study sample is classified according to the perceptual speed (48 high perceptual speed, 51 low perceptual speed), the students also are divided into three group according to screen size of mobile learning` tools (laptop, net book, mobile). Findings indicated that there are statistical significant differences between means of instructional technology students` degrees of cognitive achievement before the experiment and after the experiment in favor of post experiment, there are no statistical significant differences between means of instructional technology students` degrees of cognitive achievement in three groups relevant to screen size (laptop, net book& mobile). There are no statistical significant differences between means of instructional technology students` degrees of cognitive achievement in two groups relevant to perceptual speed (high perceptual speed vs. low perceptual speed. Consequently, there are positive correlation between students` perceptual speed and students` degrees of cognitive achievement.