This study aimed to investigate the effects of integrated intensive and extensive reading instruction (IIERI, hereafter) on Thai primary school students' English reading ability and to find out the students' opinions towards the instruction. Eleven Thai students studying at the primary education level were purposefully selected to participate in this study. The students took a pretest in the first week; then, they received the implementation of the integrated intensive and extensive reading instruction (IIERI) for ten weeks. They were subsequently administered a posttest and were interviewed individually at the end of the study. The quantitative results of the students' English reading ability in the English reading ability pretest and posttest revealed a statistically significant difference of p < 0.01, indicating that the students' English reading ability in terms of their English vocabulary knowledge and their English reading comprehension improved significantly after receiving IIERI. Furthermore, the qualitative results obtained from the interviews revealed the students held positive opinions towards IIERI: the students enjoyed receiving IIERI; the students saw the improvement in their English language knowledge and skills as a result of receiving IIERI; the students' reading habits changed after receiving IIERI; and the students recognized the benefits of IIERI.