In the information era, data security is a big problem troubling countless computer users. If we want to furthest protect important data, we should know threats to data security at first, and then take corresponding defensive measures. Firstly, let’s see threats to data security: - Hard disk or storage device suffers physical damage because of users’ inappropriate use or natural disaster. - Hard disk failure or hard drive faults or system crash. - Sudden power outage. - People delete or format data by mistake. - Data are deleted maliciously by other people. - Virus or hacker invades computer. If important, private or confidential data get lost, users may suffer great spiritual losses or even serious economic losses. Therefore, it is quite necessary to take corresponding defensive measures. In this article, main measures are listed below: Firstly, install reliable anti-virus software to prevent virus and hacker from invading our computer. Secondly, back up important data regularly. Mac users can open Time Machine to back up data timely. Thirdly, start and shut down computer normally. Fourthly, set password for computer or encrypt significant data. Fifthly, be cautious about every operation when using or managing important data. As long as users take the above measures in daily computer use, data security can be ensured largely. Of course, if you have other good ideas of data protection, please share them with us.