Car or motorbike?

I'm planing to live in nepal kathmandu for 1 year, which is best in kathmandu road??

You can't be careful enough on nepali roads. Accidents happen only too frequently. Again, a motorcyclist was just reported to have been killed in Kathmandu near Singhadurbar when he was struck by a taxi. The victim was 21.

I absolutely prefer motorcycle transport.  I drove for 2 years and the only accidents I had were just me being clumsy (no injuries).  You have to keep your head on a swivel, which is true of every place you go on a bike.  Besides that, to see Nepal from anything other than on a bike is a lesser experience.  Owning a bike is pretty easy too and when petrol is short, you will be ok.  There is a bit of a process in buying one but it's doable.  On a bike you can avoid most of the terrible Kathmandu traffic.