Hi, I am the non- custodial father. I pay child support every month. In my ex girlfriend allows her mother to claim my daughter. What can I do about this.

You are clearly the non-custodial parent, so, unless you get written consent from your girl friend to claim the child, or you have some type of separation agreement IN WRITING that allows you to claim the child, you are pretty much out of luck.

Even if you can show you are providing more than half of the child's total support (which, BTW, is NOT likely if you are NOT the custodial parent), you must have the cooperation of the custodial parent for you to claim the child.

You can do your own research by reading Chapter 3 of IRS Pub 17, which can be downloaded from Internal Revenue Service.