Understanding and manipulating the behavior of oxygen vacancy in oxide materials are of vital importance for rejuvenating materials with novel functionalities. We herein report a exciting phenomenon of oxygen vacancies changing from an isolated state to a clustered state in LiNbO 3 single crystals. The clustering of the oxygen vacancies induces a relaxor-like dielectric anomaly and a first-order phase transition. The relaxor-like dielectric anomaly was argued to be a pseudo-relaxor behavior resulting from the combined contributions of a dipolar relaxation and a Maxwell-Wagner relaxation. The first-order phase transition was ascribed to be an electric-ferroelectric phase transition. Interestingly, a well-defined melting point of the oxygen-vacancy clusters was observed. At temperatures near the point, a small dc field can lead to resistance switching from a high resistance state to a low resistance state, yielding a prominent memristive effect with the OFF/ON ratio of 10 2 . Our results underscore that controlling the oxygen vacancy state is a promising strategy to tailor the properties of oxides for novel device applications.