The grounds for an annulment are:

• One of the parties was already married to someone else when the marriage took place

• Willfully not consummating the marriage

• Not consummating the marriage due to incapacity

• Mental illness that makes the person not fit for marriage

• The spouse is pregnant by another man

• If it can be proven that the spouse had a communicable venereal disease before the marriage took place

这个比离婚要彻底，因为annulment的法律效应是把结婚状态打回到这段无效婚姻之前，也就是说在法律意义上，这段婚姻从来没有发生过。如果你这段婚姻之前没有结过婚的话，那你在法律上就是Single, Never Married!

An annulment is a legal procedure to dissolve a marriage, declaring it null and void. However, unlike divorce proceedings, an annulment works on the basis that the marriage never properly existed in the first place.

(http://www.singaporedivorcelawyer.com.sg/annulment/)