This study aimed at understanding the curriculum decision and instruction implementation of mathematics remedial instruction (MRI) in elementary schools. Classroom observation, interview, and survey data were collected from 8 case teachers, and 300 teachers who taught the After-School Alternative Program (ASAP) in Kaohsiung and Pingtung County of Southern Taiwan, respectively. Analyses of classroom observation and interview data indicated that the main foci of MRI were conceptual understanding and algorithmic practice. Teachers demonstrated how to solve an example task, and then provided similar tasks for students to practice while providing individual guidance for students as needed. Mastering mathematics skills was the primary goal of MRI. Teachers expressed that they diagnosed ASAP students' mathematical competence before selecting MRI curriculum. However, due to the diversity in students' competence levels, they chose the original mathematics textbook as teaching materials. Teachers also struggled with addressing the different learning needs of students, and they needed more time to implement individualized instruction. Survey responses revealed consistency with findings from observations and interviews. About 80% of teachers reported difficulties in decisions related to curriculum materials and instruction implementation of MRI. In the future, researchers should develop curriculum materials and instructional strategies of MRI that assist teachers in implementation of teaching materials and methods.