Thermally grown tungsten oxide (WOx) thin film on a tungsten electrode is studied as a potential memristor material. Due to the dislocation formation inside the film, the device could present stable memristive characteristics without electroforming process. In this research, the opportunity of applying the WOx memristor to the multi-level resistive switching behavior based multi-level memory or artificial synaptic device was presented. In particular, the flexibility of the device shows the possibility of a future flexible memory device. Interpreting the memristive characteristics, the resistance switching mechanism was discussed based on the valance change at the device structure interface. Based on the simple fabrication process and electroforming-less operation, the results present the possibility of applying WOx based memristors in current electronic devices at an early stage. This work reveals a useful proposal of WOx based memristors for emerging device applications.