This experiment focused on 11th Graders’ mastery level using Strategic Intervention Material (SIM) based-instruction in General Mathematics least mastered competencies. Pre/post test Quasi-experimental research was employed with two classes of 11th Graders from the Technical, Vocational and Livelihood(TVL) track taught via SIM-based instruction and direct instruction, respectively. These learners took the subject amid the school year 2018-2019 and are non-numerates in their previous mathematics class. To ensure proper maneuvering of the SIM and to ensure time-on-task the teacher-researcher handled both classes at the same venue and time frame. The learners’ pre/post test mean scores served as basis in determining learners’ mastery level while t-test analyses tested the difference of the pretest and post test scores of the two groups. Findings revealed that the post test mean scores of SIM-group is higher than the DI-group, hence SIM-based instruction is an effectual means in reaching mastery on least mastered competencies in General Mathematics. Based on the results of this study, it is recommended not to delimit Direct Instruction in upskilling least mastered competencies alongside with SIM-based instruction. Furthermore, SIM-based instruction can be implemented to other schools to supplement its effectiveness and to maximize its use in the future, and that similar studies on the use of SIM-based instruction can be conducted for remediation in other learning areas to confirm the result of the study.