Studies of errors in mathematics are essential for mathematics educators to design and contextualize a whole new instruction accordingly. Nonetheless, less attention has been given to the mathematical writing errors when compared to the mathematical conceptual and procedural errors. It is mainly because the former mistakes usually do not affect the final answer or are seemingly irrelevant to students' mathematics knowledge. Most of the studies surveying the mathematical writing errors focus on the undergraduates, followed by middle school students. Little has been done towards the primary school pupils. This paper aims to identify and classify the types of mathematical writing errors committed by primary school pupils in three domains: number and operation, measurement, and statistics. A qualitative approach is employed to identify and classify the mathematical writing errors committed by 29 above-average students from three schools. The results show that six categories of mathematical writing errors are observed. Lack of term or phrase and misuse of mathematical symbols top the overall categories of errors. The close-up views of the solutions with written errors provide some insights to tackle the problem by the mathematics teachers starting from the tender age of the students.