The calculated distance (air line) between Niš and Sremska Mitrovica is approximately 161 miles respectively 259 kilometers.
By car or train, the actual journey to Sremska Mitrovica is certainly longer, as only the direct route (as the crow flies) between Niš and Sremska Mitrovica has been calculated here.
Air line (approximately)
259 kilometers
140 nautical miles