The purpose of this study was to determine the effect size of instructional strategies on students’ research skills in Marilog District, Davao City Division. This study used the descriptive-comparative research design. The respondents of this study were 74 senior high school students from the Marilog District of Davao City, City Division. The researcher used Tabachnick and Fidell's (2007) formula, n = 50 + 8(m), where m represents the total number of independent variables included in the study, to select the respondents. The gathered data were analyzed and interpreted using statistical tools, including frequency and percentage, mean, One-way ANOVA, and Eta-squared. The results revealed that teachers predominantly use inquiry-based strategies in their teaching. Moreover, on the level of students’ research skills, in terms of writing and reporting results, and problem conceptualization, were high. In terms of research methods and data analysis, they were found to be moderate. Furthermore, it was found that there is a statistically significant difference in students' research skills when exposed to instructional strategies. Lastly, regarding the effect size of teachers' instructional strategies on students’ research skills, the results indicated a large effect size. Thus, it is recommended that teachers should refine and consistently apply inquiry-based instructional strategies in their classrooms. This instructional strategy enhances students' effectiveness in fostering critical thinking, problem-solving, and data analysis.