The aim of this paper is to use the mental models theory to study some problems of deontic logic, namely Jørgensen's dilemma and Ross' paradox. In the first part, after some brief remarks about cognitive semantics, a general description of the mental models theory is given. In the second part the application of the mental models theory to normative sentences is described. Finally, in the third part, Jørgensen's dilemma and Ross' paradox are described and then analyzed through the mental models theory.