The plane was flying in bad weather. It lost contact 33 minutes after take off.
On board were 15 passengers and three crew members. It crashed in a forest in the district of Arghakanchi in northwestern Nepal.
The agenda said the Twin Otter craft operated by Nepal Airlines took off from the western lake city of Pokhara, and was on its way to Jumla in the north.
Security personnel were being deployed to the area. Another flight with search crew could not land because of bad weather.
In the past three days, Nepal has been hit by heavy snowfall and rain.