Seeing from http://www.asiabizsetup.com/singapore-personal-taxes-qa.aspx, tax for EP is the same to Singaporean as shown in that table. But sb told me EP and PR should be taxed 15% of total annual income no matter howmuch they earn. Which one is true? Thanks for ur answer~

You will be treated as a tax resident for a particular Year of Assessment (YA) if you are a:

Singaporean; or PR or Foreigner who stayed/worked in Singapore for 183 days or more in previous year (excludes director of a company).

well, if you're a singaporean or PR but stay less than 183 days in singapore in a calendar year, you may appeal to be treated as a non-resident (if you think you may be able to pay less tax under that scheme).

For example, I start my stay&working in Sing since Jan 2008.
If the number of days from Jan to my 'tax month' is less than 183 but I`ll go on living in Sing, I`ll also be taxed 15%

Sorry,I never heard the term of 'tax month'.
Based on what I know, your 2008 income will be taxed in 2009.
If you stay more than 183 days in singapore in 2008, you will be taxed as a resident. But if you stayed less than 183 days in singapore, you maybe taxed at 15%(or resident rate depends on which one is higher). But if you can prove to the authority you have left singapore for certain reason(i.e, send overseas to do projects by your company), you may still be taxed as a resient.
But frankly saying, normally IRAS won't do such detailed checking to individuals.