The purposes of this research were: 1) to develop reading comprehension skills exercises based on supplementary reading books of Thai Language Department for Prathomsuksa 5 students to be effective according to the 80/80 criteria and 2) to compare the students’ reading comprehension skills of both pre and post learning with the supplementary books of Thai Language Department for Prathomsuksa 5 students. The sample group used in this research was a group of 30 Prathomsuksa 5 students from Banklongkrok School in the first semester of the academic year 2020 selected by purposive sampling. The research instruments were: 1) seven supplementary reading books including (1) a Chanthaburi folk tale on Phranang Kawai, (2) a fable on Phraya Chang Phu Siar Sara, (3) general tales on Ma Nua Mek, (4) Wan Lok Taek, (5) Pak Pa Suay, (6) Khae Phrip Ta Diaw and (7) Thay Thod Sod, 2) reading comprehension exercises, 3) lesson plans and 4) a reading comprehension test. The statistics used for data analysis were: mean, standard deviation and T-test.
