The calculated distance (air line) between Luang Namtha and Xam Nua is approximately 175 miles respectively 281 kilometers.
By car or train, the actual journey to Xam Nua is certainly longer, as only the direct route (as the crow flies) between Luang Namtha and Xam Nua has been calculated here.
Air line (approximately)
281 kilometers
152 nautical miles