Thanks for your information.
I use PC for a long time but become a Mac user just few days ago. So I get lot things to learn.
When I use PC there is a wipe software called “R-wipe&Clean”. It is a complete solution to wipe useless files and keep your computer privacy. Irretrievably deletes private records of your on- and off-line activities, such as temporary internet files, history, cookies, autocomplete forms and passwords, swap files, recently opened documents list, Explorer MRUs, temporary files, etc.
I am looking for the same tool for Mac. After searching on internet what I get is two software - NetShred X and ShredIt X. If these are necessary if I use OS X's Disk Utility?