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.
While that is technically a no fee system...
1) everyone still pays transaction
costs (if not fees) in the form of hashes
2) hashes are one use only so that payment never recirculates, wasted heat
3) you'd need Brunic's hash algorithm (above) to prevent spammers from using GPUs
4) drained smartphone batteries
5) coins are just portable hashes anyways