When writing an appreciation letter to a co-worker, mention the specific event or project to which they contributed in the first paragraph of the letter. Express how his contribution has helped you or the company and that you look forward to working with him again in the future.
Keep a letter of appreciation concise and to the point. An appreciation letter should be no longer than one page in most cases. Choose a professional font, such as Times New Roman, Arial or Calibri, with the font size between 10 and 12 points. Format the letter using single-space paragraphs and leave a space between each distinct paragraph. The page layout should include 1-inch margins all the way around and text should be aligned on the left side of the page.
Address the letter directly to the person in the salutation and add a date if necessary. Use a formal closing, such as "Sincerely" or "Best Regards," and add a signature line. Reread and edit the letter for clarity and to ensure the finished product is professional. In most cases, send a physical copy of the letter to the co-worker. However, if an email is the best available option, keep the subject line simple, such as "Thank You" or "In Appreciation."