Active readers are effective readers. In Taiwan, reading instruction has taken the majority of time and energy in the English class; however, it is generally agreed that students are deficient in reading ability somehow. Thus, several solutions are raised to tackle this problem, and taking marginal notes is recommended as one of the solutions. This study, based on 44 college students, attempted to examine the effect of taking marginal notes on improving learners’ reading comprehension. The participants were given instruction in writing marginal notes, and then the effect of this instruction and learning was examined by way of their writing marginal notes in the midterm and finals. Several issues were discussed in this study. What was a marginal note? Why was taking marginal notes important in the English reading class? How could this study skill be taught plausibly so that students were able to handle it? Apart from these issues, learners’ reflection and attitude toward taking marginal notes, the difficulties of applying this study skill and the solutions to these difficulties were taken into consideration. The results suggested that the students thought taking marginal notes was time-consuming, difficult but rewarding as well as important. With the help of such a study skill, the students will hopefully improve their reading ability. Most importantly, their experience of learning to take marginal notes will help to cultivate their active reading habit and then assist them in becoming lifelong learners in their careers of learning English.