A recommendation letter is critical to any graduate school application. Likely, you will require at least two letters, and it may seem hard to determine the person to write the letters. However, after a successful application, you should write him or her a simple thank you letter to express your gratitude. Writing recommendation letters is sometimes one of the most laborious tasks for most professors. Remember, there are many students who request recommendation letters from them. You can stand out from the crowd and write a proper thank you letter. Below are some of the guidelines on how to thank a professor for writing a recommendation letter.