The relief in the administrative territory is extremely uneven, specific to the mountain area with gorges on the course of the two Jiu (Eastern and Western Jiu). Mountain ranges bordering the basin are: Vâlcan to the South, Retezat to the North, Parâng and Godeanu to the East to West, the maximum altitude of 2,519 m being in Parângul Mare Peak. Along the Eastern Jiu a meadow and terrace were formed, the latter having a smooth surface and being a little fragmented. Because it is an intermediate step between the foothills and the meadow, it was valued by the agricultural crops and its presence along the main communication routes.
The elongated shape of the city and its isolation by high mountains, have a great influence on climate issues, because the movement of the air masses is from north to south through the transverse cracks Băniţa-Merisor and Surduc-Lainici. The mountains stop the movement of the air masses, so that they stop the air refresh in the depression. Because of that and of the industrialization, the streets, the trees and the grass in the city took a specific aspect of large coalmining regions. City landscape can be compared to that in the Ruhr, where stagnant fog covers the city and stays over because of the surrounding peaks.
The first signs of the people presence in this region are since the Paleolithic era, and then the shepherds of free Dacian period, but the first inhabitants of the nowadays settlement may be considered 20 serfs from Petros , settled in Petroseni in 1640.
Petrosani official story begins in 1788, when the name of the village is recorded in the book “Journey from Potsdam to Constantinople,” by the Prussian officer Gotze. Only in 1818 the name of Petrosani is mentioned in official documents, attesting it in the census with 233 inhabitants, whose main occupation was shepherding.