In this letter, the relation between the thickness of SiO2 and the behavior of forming process has been investigated. During room temperature forming process, the resistive switching (RS) behavior was activated in the thin SiO2 (12 nm), but failed in the thick SiO2 (23 nm). The behavior of activation failure is attributed to the reformed SiOx layer due to the unexpected recombination of excessive oxygen ions and oxygen vacancies near the TiN/SiO2 interface. However, RS behavior can be presented successfully owing to the enhancement of driving ability of oxygen ions into TiN by employing high temperature forming process.