This study aims to develop performance assessment tools for mathematics in the primary school. In order to achieve this aim, it reviews the tics in the primary school. In order to achieve this aim, it reviews the meaning and the purpose of mathematics performance assessment, and the characteristics of performance assessment tasks. Then the framework for portfolio developed in this study is introduced. This portfolio is called 'mathematical thinking and applying'. It aims at balanced assessment for improvement of mathematics instruction. It is composed of journal writhing, problem by the student, constructed task, work samples, written test, self assessment, teacher's comment and parents' comment. The criteria of performance tasks is categorized in impact, reasoning, accuracy and communication. The procedures of development of these tasks are as follows: the analysis of mathematics curriculum for the primary school, the design of performance tasks with considering teaching unit goals, designing rubrics, discussing these tasks with teachers in primary school, modifying them when is needed, observing the process of children's task performing, interviewing with teachers and final modifying. After performance assessment tasks are implemented, the answers by the students is analyzed using rubrics. Then anchor papers are selected. Also, the errors of children are analyzed. Through the process, teachers can obtain the information of children for improvement of mathematics instruction. Finally in order to generalize this study, I suggest that we need to cooperate with the field of education and to establish expert assessment groups.