The study was intended to develop and establish professional standards of mentors. The main methods of collecting data included interview and questionnaire survey. Four stages the study went through were: Literature reviewing, guided interviewing for extending the framework of the professional standards, questionnaire survey in line with factor analysis used for selecting and categorizing the items, and panel discussion of revising and constructing the standards. The finalized 86 items in total included 14 sub-dimensions: professional beliefs, professional development, reflection, curricular and lesson plan, mathematics content knowledge, pedagogy, students’ learning, assessment, evaluation of curriculum and instruction, mentoring in curricular and lesson plan, in mathematics content knowledge, pedagogy, in students’ learning, assessment, and in evaluation of curriculum and instruction as well.