“Expression of intention mistakes” concerns the respect for the intention declaration of the declarer and trust in the opposite party, and is an important issue in the Civil Law, thus, the situations to be accepted as “expression of intention mistakes” for cancellation of the declaration of the declarer is worth discussing. “Expression of intention mistakes” is referred in Article 88 of the Civil Law. Both scholars and juridical practitioners suggest that the first item should be “content mistakes” and “behavioral mistakes”, while the second item should be “mistakes of the nature”. However, the discourses on the definitions are vague. Scholars held different views on the scope of “motive mistakes”. Based on the German general principles, scholars of Taiwan suggest that”mistakes of the nature” is “motive mistakes”. According to Clause 2 of Article 88 of the Civil Law, it can be cancelled on exemption. Some scholars suggested that “characteristics mistakes” was originally “content mistakes”, and Clause 2 of Article 88 of the Civil Law is merely an instruction. This paper attempts to clarifythat”content mistakes” occur on the “result meaning” of the subjective factor of the “expression”, and “behavioral mistakes” occur on the behavioral part of the objective factor. The definitions of the two can be clearly differentiated. And this paper argues that the nature of the “characteristics mistakes” is “content mistakes” in Clause 2 of Article 88 of the Civil Law. This proposition expands the identification of the scope of “content mistakes”. The cancellation conditions of “expression of intention mistakes” are different than those in Germany, namely, the declarers need to be proven non-fault. The non-fault condition reasonably restricts the cancellation of “expression of intention mistakes”. This paper explains the safety of non-improper hazardous transaction, in order to clarifythe scope of mistakes.