This paper studies the nuclear proliferation in a non-proliferation regime using Iranian nuclear stand-off as case illustration. It seeks to find out the core reasons why nuclear proliferation has been possible under international prohibition. It seeks also to find out the reason why Iran has successfully defied international sanctions and isolation against its nuclear programme. With the aid of documentary method of data gathering and rational actors model as framework od=f analysis, this paper observed that struggle for hegemony among the super powers, pursuit of international trade in nuclear materials and technology, skewed provisions in the principles of NPT, nuclear states refusal to disarmament are the major factors responsible for nuclear proliferation under NPT regime. The paper also observed the same factors together with Iran’s strategic location and natural resources endowments are responsible for Iran’s successful defiance of international sanctions against its nuclear programme. It is therefore recommended that all nuclear states should unconditionally dismantle their nuclear weapons and facilities under unrestricted supervision of the five permanent members of UNSC. The principles of NPT should be reviewed and fundamentally restructured.