Promotion letters are of two types: job promotion letters and business promotion letters. Generally the latter is referred to as promotional letter and is quite different from a job promotion letter.
Writing a job promotion letter can be a pleasant task as it is the bearer of good news. Make sure to include your congratulatory lines in the first paragraph. Mention the job title the employee will now hold. The tone of the letter should be positive and encouraging.
In the body of the letter, mention any additional perks or benefits the employee may enjoy in this new position. While lauding the employee you can also quote one or two special achievements which have worked in their favor when considered for promotion. This can help boost their morale and confidence.
While concluding you can encourage the employee to keep up the good work. Give assurance of your support and cooperation in all matters. You can sign off by giving them your best wishes in the new position.