There is little data available about the eastern parts of the territory of the great kingdoms of ancient Iran, especially the pre-Sasanian kingdoms, and therefore researchers always face more difficulties in understanding the territory of these kingdoms in the east. There is a report by Ammianus Marcellinus from the 4th century AD that describes the territory of his contemporary Persian kingdom, the Sasanian Empire, but the lack of consistency of his report with independent data and some inadequacies in this report have caused researchers to refrain from trying to benefit from the details provided by him; However this unique report can make an important contribution to the reconstruction of the Achaemenid territory in the northeast. The main question of the present study is: What data does Ammianus' report on the Scythians of Central Asia provide about the Achaemenid territory within these lands? The findings obtained from the content analysis method will show that Ammianus' report on the northeastern lands of the Persian kingdom should be considered related to the Achaemenid realm, and the two lands of "Saca" and "Scythia beyond Mount Emodes" in this report both represent the northeastern lands of the Achaemenid realm, which have been introduced as lands of the Sasanian realm due to being taken from different sources. Ammianus knows the extent of the Persian kingdom in the northeast as far as the Arimaspian/Pazyryk lands, and similarities can be found between the details of his report and some sources from the Achaemenid period, which help to better understand the Achaemenid territory in the Scythian lands. The data collection method in this research is library.