The calculated distance (air line) between Luang Namtha and Luang Prabang is approximately 88 miles respectively 141 kilometers.
By car or train, the actual journey to Luang Prabang is certainly longer, as only the direct route (as the crow flies) between Luang Namtha and Luang Prabang has been calculated here.
Air line (approximately)
141 kilometers
76 nautical miles