Teaching a mixed-ability class at undergraduate level is challenging since lecturers have to give equal opportunities to all students to develop their knowledge and skills. A preliminary study was conducted in an ESP class of Information System undergraduate program at Ibrahimy University Situbondo. The 23 students' reading ability levels vary from the low readers who are still struggling on vocabulary to the advanced readers who are ready for academic reading. The lowest student's score was 24/100, and the highest was 92/100. Only 39% or 9 students achieved the standard (the minimum grade 60). The lecturer used the same method and a reading text for all students in the previous teaching process, so it failed to accommodate all levels of the students. Based on the preliminary and previous studies, the best solution for this class problem is Differentiated Instruction (DI) approach, which then focused more on a method called tiered assignment. It is a method of teaching diverse students with the same skill or concept but grouping students into tiers and giving them assignments with different levels of complexity based on their readiness. According to the benefits of this method on reading comprehension in higher education reported in previous studies, this current study aimed to improve undergraduate students' reading comprehension in mixed-ability class using tiered assignment. The design of this study was classroom action research. A set of lesson plan was prepared for the action. The observing stage or data collection was conducted using three instruments; test, questionnaire, and observation checklist. The criteria of success were at least 80% of students achieve the minimum grade 60 and the students' feedback toward this lesson is positive. The reading scores were analyzed using percentage formula to calculate the proportion of students who achieved the standard. Two cycles were conducted. The results showed that 91% of students could achieve the minimum standard and gave positive feedback toward the learning process. It implies that tiered assignment can improve reading comprehension in this class with a set of modified scenarios. The key strategies that make this action research successful lie on; 1) combining the differentiation of pre-reading material, teacher assistance, and reading assignments; 2) designing the materials and assignments for each tier in the same portion and duration; and 3) applying classroom management in which the teacher provided full assistance on the basic tier while keeping track of students progress in the other tiers