Since postwar period between Silla and Tang composed of allied force, Na-Tang War occurred. internal and external academic world see this issue very differently. Korea academic grasp major cause of Na-Tang war is that Silla and tang had a secret treaty about territory separation at post period time but tang broke a deal. External academic bring military activity of Silla into question. Because of this difference points of view, we need check the cause of background of the Na-Tang war that was military activity of Silla or tang broke a secret treaty. In this situation, may, 669 sending first ambassador for apology to tang from Silla come to the fore significantly. Sending ambassador article is a core foundation about point of begining war as a directly war article. However, the recordings from “Samguksagi” didn`t specify why Silla send ambassador for apolgy to tang. it is only assumed that reason on the authority of the first month of the year, 670 article that Silla was taken territory and the people of Bakjae. By contrast, “Samgukyusa” is recorded Gojong inquire ambassador of Silla for murder case of Tang army at that time. This article take note of this difference and compare with relative record from “Samguksagi” and “Samgukyusa”. Based on these record, we can check the reason why Silla send ambassador for apolgy to tang and what is a Tang response to this. Lastly, this article check crucial reason why Na-Tang war occurred through these record.