Paying extra toward the principal will not lower your interest rate, but it will lower the interest you pay in total. For example, if you're paying interest on $100,000 currently, and you pay a lump sum of $15,000 toward the loan, then you will only pay interest on $85,000 going forward. Before you make that lump sum payment, please make sure that your mortgage company allows this and how you can make sure that it is applied correctly.