This study aimed to investigate the effectiveness of Prospect Hill's Model (PHM) of assessment portfolio on mathematical writing, mathematical thinking and achievement on grade nine students in the Sultanate of Oman. Study sample consisted of 64 female students divided into two groups each of 32 students. The first group is experimental and used the PHM of assessment portfolio. The second group is a control group and used the ordinary work portfolio that is applied in the Sultanate's schools. The equivalence of the two groups was checked. The study took 6 weeks, with 7 weekly lessons. After completion of the study, three tests prepared by the researcher in mathematical writing, mathematical thinking and student achievement were given to the students of the experimental and control groups. Results of the study revealed that the difference between the two means of the experimental and the control groups is statistically significant in the three tests and in favor of the experimental group, which indicates the positive impact of the assessment portfolio model on mathematical writing, mathematical thinking and student achievement. As a consequence of these results, the researcher presented some recommendations and suggestions the most important of which was teacher's training to apply the model of the assessment portfolio, carrying out the study with some other variables such as creative and critical thinking and attitude toward mathematics.