Slander

Yes, it is possible that your father could sue you for defamation, but whether he would have a case or not depends on alot of circumstances.  First, what is the written statement for?  There are certain immunities that might apply if you are making a written statement as part of testimony in a court proceeding or as part of an official complaint.  Second, the written statement has to be "published", this means that you would have to actually communicate the contents of the statement to another person.  Third, usually truth is an absolute defense to a case of defamation.  This means that if the statements you make can be proven, and are true, then you have an absolute defense to a defamation claim.  You should remember though, that the above only addresses the liklihood that his claim will be successful.  There would be nothing stopping your father from filing the lawsuit and dragging you through a defamation claim even if his claim had no merit.  So, yes he can sue you for defamation, but depending on the circumstances, he may or may not be successful.