The purpose of this study is assessing the impact of CLIL based curriculum on students' integrated skills in ESP. To achieve the study objectives the researcher used a quantitative design comprised (30) first-year secondary students in a quasi-experimental design to measure the effect of teaching an entire module from a CLIL based EFL curriculum on their ESP integrated reading and writing skills. The researcher employed a pre-posttest and test rubric for ESP integrated skills assessment. Results showed that the CLIL based curriculum has an effectiveness which was confirmed through significant improvement in students' ESP integrated reading and writing subskills post implementation of the curriculum at a significant level of (0.01). Notably, the integrated reading and writing subskills had the highest effect size (1.57), followed by ESP reading subskills (1.53), and finally ESP writing subskills (1.33). Based on these findings, recommendations were outlined focusing on enabling secondary students to acquire subject-related content, cultural awareness related to language use, cognitive aspects, and communication skills through application of CLIL principles.