Given the difficulty of teaching Cambodian learners the English speaking skills due to large, mixed-ability classes, and inappropriate applications of existing resources, the approach of Task-Based Instruction (TBI) was tested with ninth-grade students. This article, therefore, investigates the effect of TBI on ninth-graders' English-speaking skills and their satisfaction toward the experience with this approach. The pretest-posttest non-equivalent quasi-experimental group design was utilized with two Grade 9 classes totaling 78 students. Both quantitative and qualitative data were collected by using the speaking tests and the student satisfaction questionnaire. Quantitative and qualitative analyses indicated that the TBI made significant contributions to the experimental group's speaking skills in general as well as in all sub-skills either when comparing the pretest and posttest within the group or the posttests between groups. Analyses of the satisfaction questionnaire found that the experimental group was 'satisfied' with their experiences with the TBI. Students also viewed TBI as an approach that provided them appropriate conditions for language learning, helped improve their speaking skills, increased their confidence in speaking, and motivation in learning English despite the difficulty with the language of instruction. Based on these findings, some recommendations for pedagogical implications and further research have been proposed.