At the moment, I was doing my dissertation about level of customer service toward hotels. So I need to write letter of permission to the Human Resource department enable me to interview some staffs or managers, as well obervation studies towards the service provided by staff to guests. But I don't know how to write and where to start? Please help me n Thank you very much, if you able to answer my question.

Here are a few examples:

Sample Letter of Permission (http://www.carrollcc.edu/library/research/copyrightinfo/letter.asp)
http://www.copyright.iupui.edu/_permitintro.htm
http://www.wikihow.com/Write-a-Letter-of-Permission
http://www.ametsoc.org/pubs/copyrightinfo/ams_model_permission_letter.pdf
How To Write a Letter of Permission | eHow.com (http://www.ehow.com/how_2273591_write-letter-permission.html)

You could also post a draft of your letter here. I'm sure that there will be people who will come along to critique it and give you suggestions as to how you might make it better.