The packed bus was ferrying passengers - who had been celebrating the Hindu festival of Dashain - from Sindhupalchowk to neighbouring Kathmandu.
But the crowded vehicle slipped and fell more than 50 metres at a bend.
21 pilgrims die, 35 hurt in India bus crash
"Six people were killed instantly and five more passed away on the way to the hospital or while being treated," district official Goma Devi Chemjong told AFP.
Chemjong said 108 people were injured but only 39 were kept for treatment.
"We are investigating what caused the accident," she said.
Accidents are common on Nepal's poorly maintained highways, with the number increasing during the festival season when the roads are far busier and public transport overcrowded.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ