Abstract
This study aims at developing a valid, practice and effective set of mathematics learning materials based on Numbered Heads Together (NHT) model with peer-assessment to enhance students' conceptual understanding and achievement. To fulfil the goal, the research was carried in research development setting using ADDIE model (Analysis, Design, Development, Implementation, Evaluation). There are two products in the set of learning materials, which are lesson plans and student's worksheets. To ensure the data validity, the developed products were validated by two experts. Afterwards, the revised products were implemented in a junior high school in Selong, Nusa Tenggara Barat-Indonesia, with 1 mathematics teacher and 32 students, to evaluate its practicality and the effectiveness. The data from teacher's and students' assessment sheets, students' work in test and classroom observation were collected during this implementation process. The data were analyzed by converting the total quantitative data scores obtained into five scale qualitative data. The results showed that the instructional package developed were valid, practical, and effective to improve both students' mathematics conceptual understanding and achievement.