The study aimed to develop Strategic Intervention Materials (SIMs) and investigate their effectiveness in teaching the least mastered skills on subject-verb agreement. The researcher utilized an experimental research design under the quantitative method. The study’s respondents were 30 Tayawan National High School, Bayawan City students. The researcher utilized validated questionnaires and employed mean, t-test for dependent data, weighted mean, and Spearman’s Rank Order Correlation Coefficient in treating the data. The study revealed that the pre-test performance of the students has “not met the expectations” while their post-test performance is at the “fairly satisfactory” level. It is also revealed that there is a significant difference in the pre-test and post-test results of the students in the following areas: compound or singular subjects joined by ‘and’ are always plural, indefinite pronouns, collective nouns, and the expressions, “the number” and “a number.” Moreover, the students perceive the effectiveness of the SIMs to a “very high” level, which is significantly related to their post-test performance. The use of SIMs was a successful intervention that helped students perform better on the posttest