Hello,
I am selling web hosting packets on my sever and i want to limit user downloading folder. İ want to limit downloading limit to 300k for every person . for instance if there is 100 visitors and i want to limit each of their downloading limit to 300k . every single visitor should only download 300k of disk size within 1 sec..

You can use PCQ and set limit 300k for each client,
http://wiki.mikrotik.com/wiki/PCQ_Examples
i just want to limit visitors (for each client on unique IP) that using one machiner , it is not that i want to limit machines only clients on ane machine ?