With the rapid growth of digital technology, mobile phones are widely used for EFL teaching and learning. However, little is known about the effectiveness of integrating a mobile application into EFL classrooms to improve oral accuracy in Thai school contexts. An instructional intervention grounded with Mobile Assisted Language Learning (MALL) and dynamic assessment in Thai EFL contexts needs to be further investigated. This quasi-experimental study aimed to investigate the effectiveness of a mobile-based intervention to develop Thai EFL learners' speaking. Thirty students from a secondary school in southern Thailand were assigned to complete five oral tasks through the dynamic assessment process as an intervention on WhatsApp in order to develop their oral accuracy. Quantitative data were collected from the oral pre-test and post-test to examine the students' speaking development by utilizing the paired samples t-test. The results showed a significant difference between the pre-test and post-test after the implementation of the MALL-based intervention. It could be suggested that the application of the MALL-based intervention could develop EFL learners' oral accuracy in Thai school contexts. Implementations and recommendations of this study are discussed.