The notion of transgression although not seemingly as a key word in Lacan's nomenclature; however, it harbors abundant connotations associated with a cluster of his peculiar coinage and appropriation.Starting from a contour of Foucault's transgression,we do disclose the lacunae for his very attempt from ethics to ontology. This paper purports to delve into the context of the ethics of psychoanalysis from combing the clues in the Freudian origin, sorting out the "calming of guilt" derived from auto-punishment paranoia in their twists in order to cope with "sense of obligation"attributed to the terrain of morality in the framework of confronting the pleasure principle versus the reality principle to their termination. Following this thread, it's of vital importance to divide the imposition of moral law buttressed by the incest prohibition (the Good) from drawing on a particular interest (the good), both of which are surmounted in the respect of transgression. In recourse to further renounce the precarious couplet, its excess and exterior are intimately entwined with the detached objet a in the formulation of the fantasme. The final twist comes up with its reverse per se to examine on the structure of perversion. Our conclusion is concise and open: the impossible jouissance in Lacanian context should be totally differentiated from the perverted detour, for the latter is still confining to the transgression of the law (the castration), engendering paradoxically a certain infraction but just can't trespass it.