You might write your boss a letter about a fellow employee for a variety of reasons. Perhaps you find your teammate's efforts and initiative to be outstanding and want to recommend her for an award. On the other hand, she may be harassing you or sabotaging your work efforts to such a degree that you need your boss’s intervention. Regardless of the reason for the correspondence, be honest and straightforward, and don't put anything in writing you wouldn't be comfortable repeating in person.
Think Before You Write
Before you put pen to paper, you should think through anything you have to say to your boss about a fellow employee. Make sure what you’re saying is factually accurate and consider how your boss will look at the letter. If you're writing something in an effort to put down a colleague to bolster yourself in the eyes of your supervisor, you can do it in more effective ways, such as volunteering to take on additional responsibility and tasks. You may have to write a letter to describe unethical behavior of an employee, but make sure you have facts to back up your claims.
Introduction and Body of the Letter
Lead off your letter by clearly stating the reason for writing. Your wording following the introduction should include details that back up your assertions in the introduction. If you’re including attachments, such as printouts of email discussions, memos or other documents that support your claims, reference those in your introduction as well. For example, “As you'll see from the attached documents, this is an issue that has become a problem over the past several weeks.”
Letter of Conflict
If you have a conflict with a colleague you’re unable to resolve on your own, and you want to bring the matter to the attention of your supervisor in writing, be prepared to highlight all efforts you've made to date. State the situation as well as the outcome you want to see. For example, “For the last three months, Susan has failed to meet print deadlines, resulting in delays and additional costs for my department. I’ve attempted to talk to her about this on three different occasions, but we haven’t been able to resolve the problem. At this point I would appreciate your intervention and assistance.”
Letter of Commendation
If you’d like to bring exceptional performance or the teamwork efforts of a colleague to the attention of your supervisor, a letter is the place to do that as well. For example, “I’ve had the privilege of working with Susan for the last several months, and her performance is exceptional. She’s been a stellar team member, completes work ahead of schedule, and is always willing to lend a hand to colleagues. I'd like to outline a few of the ways she has become a major asset to our department.”