The calculated distance (air line) between Phnom Penh and Siem Reap is approximately 143 miles respectively 231 kilometers.
By car or train, the actual journey to Siem Reap is certainly longer, as only the direct route (as the crow flies) between Phnom Penh and Siem Reap has been calculated here.
Air line (approximately)
231 kilometers
124 nautical miles