Amorphous-IGZO (a-IGZO) TFTs have significantly attracted attention for high-end and large area displays, due to their high field-effect mobility, low process temperature, and high transparency to visible light. However, a-IGZO TFTs have instability issues suffered from electrical stress and environment conditions during the driving devices, resulting in large shift of threshold voltage (Vth). Recently, to resolve these instability issues, IGZO thin films having a c-axis aligned crystal (CAAC) structure were proposed by semiconductor energy laboratory (SEL) and CAAC-IGZO TFTs exhibited the excellent device performance and reliability because CAAC-IGZO structure has lower defect levels than conventional a-IGZO. Here, we had a one question whether this only specific structure having c-axis aligned crystal was available to improve the device reliability. In other words, is it possible that IGZO with crystal structure, even if IGZO do not have a CAAC structure, can be used as a channel material to enhance the device reliability? In addition, a systematic study on effects of IGZO thin films as a function of crystal structure still remains insufficient. Thus, studying the device stability against various stress conditions with respect to crystal structure of IGZO thin films can be considered with much attention. In this presentation, we studied the effects of crystalline IGZO (c-IGZO) on the device performance, focused specially on the device reliability. Before measuring the electrical properties of c-IGZO TFTs, in order to confirm the film structure according to the annealing temperature, crystal structure and microstructure of IGZO thin films were preferentially investigated. After that, the electrical properties of c-IGZO TFTs as a function of annealing temperature were measured compared to conventional a-IGZO TFTs. Finally, effects of the c-IGZO on the device reliability were analyzed under positive/negative bias stress with/without light illumination having a constant wavelength and intensity. To this end, we propose that the clear relationship between the device reliability and c-IGZO and also crystal structure itself play an important role in device reliability rather than the structure of crystal, such as a CAAC structure.