i am visiting nepal soon. i have us passport. i also have nepali passpport which is not expired yet. can i visit nepal in nepali passport or i must have to use us passsport ?

Use the US passport.

Sinope, you can enter Nepal by using your Nepali passport but coming back you need to show either US Visa or Green Card else they will not let you board the flight. Better use your US passport, apply visa for Nepal (https://online.nepalimmigration.gov.np/ ) and pay $40 fee upon arrival.

About nice Restaurants google "M&S hunger hunt", you will find plenty of options.

Why would you invite several hassles by traveling with Nepalese passport when you have already taken US passport.
Like Jonny said, going to Nepal will not be an issue with Nepalese passport but coming to the US will be an issue. If you are thinking of showing Nepalese passport to enter Nepal and while returning, thinking of showing US passport, you cannot do that. It is because there will be no entry stamp or Nepalese visa in your US passport thus it will appear you entered Nepal illegally.
Just use US passport and your visit will be smooth.

thanks max, jonny and ts for shedding knowledge.
So, I must apply for the visa online before leaving US or i can do everything there at airport upon arrival ?

Thesedays there are kiosks at the ktm airport so it's easy to apply. It's still a pain to have to stand in line to pay the fees.

You can also fill online http://online.nepalimmigration.gov.np/tourist-visa

Sinope, you can apply visa upon arrival at TIA too but the Kiosk there to apply can get really crowded. You can easily save an hour or two at the airport if you apply online and save the bar code (either print or save on your phone). Note that the bar code is only valid for 15 days so make sure to complete your online application within 15 days of arrival at TIA. I did mine only few days before my flight.
Hope this helps.

from my experience, i suggest you to use US passport and also carry USD cash amount for visa fee.

Thanks guys