Re: I wanna design a new crypt coin, what features do u think should be included in?

maybe transaction fees is 0

You need some transaction fee to prevent spam.

No, you don't. Proof-of-work prevents spam too.

but people more send very big amount of little coins to attack the whole network.

They can't. 1000x transactions require 1000x calculations to be made. If 1 transaction requires 1 minute of calculations on average CPU then not more than 1440 transactions can be sent during 24 hours.

uh, u mean one user can at most send 1440 transactions under ur condition?

Yes. It's the same as http://en.wikipedia.org/wiki/Hashcash.