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