The calculated distance (air line) between Siem Reap and Kampong Thom is approximately 83 miles respectively 133 kilometers.
By car or train, the actual journey to Kampong Thom is certainly longer, as only the direct route (as the crow flies) between Siem Reap and Kampong Thom has been calculated here.
Air line (approximately)
133 kilometers
72 nautical miles