Thai undergraduate engineering students seem to have difficulty mastering English oral communication ability. This study investigated the effects of project-based blended learning with communication strategy instruction to develop English oral communication ability of undergraduate engineering students. Four communication strategies, namely asking for clarification, asking for confirmation, circumlocution, and use of fillers and hesitation devices, were taught to 20 undergraduate engineering students inside class. The participants carried out their online tasks and independent project via social platforms (e.g., Facebook, Skype, etc.). Quantitative and qualitative data collection and analyses were conducted. The participants' pretest and posttest scores were compared using the Wilcoxon signed rank test to examine their English oral communication ability development in six aspects (i.e., range, accuracy, fluency, interaction, coherence, and pronunciation) based on the Common European Framework of Reference for Languages (CEFR) 2017. The findings revealed that changes in English oral communication ability took place after the participants were taught communication strategies, thus indicating that project-based blended learning with communication strategy instruction could be used to promote English oral communication ability of language learners.