Hi Mahdi,
I see you've posted quite a few questions. It would be more helpful if you presented us with your thoughts on the difference between the two items so we can help clarify any points of misunderstanding. For this one specifically, DfX refers to technical guidelines you would use to optimize specific aspects of design (you want to optimize 'X' - e.g. cost, reliability, quality). For DoE you are changing several variables to determine their impact on quality (changing many parameters - I think of this as trial and error).
Kyle