If you are being very strict - 'in one hour' would mean that you would arrive before an hour has elapsed. 'after one hour' would mean you would arrive after that. But then, many people would not be that accurate, so they would often be interchangeable. It would also be slightly more natural in an informal context to say 'in an hour' rather than 'in one hour'.

" I will be there after one hour" >> It means Now we have not been there yet. It takes one hour to come there. " I will be there in one hour." >> One hour is period of time that we are there. We have planed to be there for one hour.. Actually, I am learning English. So I am not sure about that.. I hope to be given clues from you. Thanks for a fantastic question.. :)