how to tell him?

[watkins]

Umm it depends on how religious your dad is. I presume from this thread that he is religous and that is why you are having issues with telling him you are an atheist. I would just ask him how he feels about you being an atheist - then have a good explanation as to why you are. If he is reasonable then he may be a little worried for your soul or whatever but should respect your decision. Good Luck!