Scholars have different opinions on the date of the composition of the Yinwenzi. This paper uses the language of the book as a tool to investigate its date of composition. It chooses five words, viz. xiongnüe 凶虐, tongcheng 通稱, canbao 殘暴, wujia 無價, and qianci 謙辭 from within the Yinwenzi and the investigation shows that they appear, beside the Yinwenzi, only in works of the Eastern Han and the Wei-Jin. It is therefore safe to conclude that the Yinwenzi available today is a forgery of the Wei-Jin period.