Regardless of the brain's limited storage capacity, teachers must exercise caution and vigilance in the classroom to prevent overwhelming students with excessive information, as this can impede their learning ability. This study aims to compare the effectiveness of traditional PowerPoint slides and Assertion-Evidence designs in improving grammatical learning among Thai EFL students. Additionally, it seeks to determine the degree to which Assertion-Evidence design based on the cognitive load principle, helps students retain their knowledge of English grammar and explore the perception of students who were given English grammar lessons using Assertion-Evidence design presentations. In this study, 59 students were randomly divided into a control group and an experimental group exposed to PowerPoint presentations using the Assertion-Evidence design concept. English tenses pretest, immediate and delayed posttests, a focus group discussion were used as instruments. The results demonstrated that students who received instruction using PowerPoint presentations that followed the Assertion-Evidence design performed significantly better on the posttest for English grammar than those in the control group. Regarding retention, the results revealed that the mean score on the delayed posttest for the participants in English grammar was statistically higher than their immediate posttest score, with a p-value of 0.043 from the dependent sample t- test (p < 0.05). In terms of students' perceptions, it was discovered that the initial design of Assertion-Evidence format influenced their positive and negative attitude in learning grammar concepts using the Assertion-Evidence format presentation.