Bùɡuān nǐde shì.

不关你的事。

It's none of your business.


Notes:

不(bù): not

关(ɡuān): be relative to

你的(nǐde): your

事(shì): matter，thing

Example:

A：你明天去参加晚会吗？

     Nǐ mínɡtiān qù cānjiā wǎnhuì mɑ？

     Will you attend the party tomorrow?

B：不关你的事。

     Bùɡuān nǐde shì.

     It's none of your business.