The present study examined the effectiveness of utilizing the CLIL-based instruction to enhance the engineering-majoring students' EFL functional writing and critical thinking skills. The 64 participants were third-year engineering majoring students in Misr University for Science and Technology (MUST). They were required to study the ESP course of Academic Writing during the first term of academic tear 2023-2024. They were equally divided into control and experimental groups with 32 students each. The present study employed two different versions of the functional writing and critical thinking pre-and-posttests. It also designed two assessment checklists of the functional writing and critical thinking skills. The current paper analyzed the elicited data quantitatively and qualitatively. The study employed descriptive and inferential statistics for the purpose of data analysis. As such, it employed the mean scores, standard deviations and frequency on one hand and paired samples t-test and independent samples t-test on the other. The findings showed better performanceof the experimental students in the functional writing and critical thinking posttest compared to the control group students. They also had higher assessment rates in terms of functional writing and critical thinking skills compared to the control group. Recommendations were based on integrating the CLIL into curriculum planning and design of teaching ESP courses to engineering-majoring students in MUST.