It lowers your principal and thereby the amount you pay out in interest, but it won’t affect your contractual interest rate. Paying down a mortgage early is an excellent idea. Many people with 30 year mortgages pay them off in 15 years. Paying down your mortgage when you’re young and at the height of your income producing years is a godsend when you’re older. Eliminating your largest monthly cash outlay will free you up to do things others can’t when you’re older, like sailing the Caribbean, or traveling in Europe or Asia. And you’ll always have a home to come to when you’re done.