This paper describes the high-field studies on accelerator structures conducted at the X-band Test Facility, XTF, which was commissioned at KEK in 2004. A 60cm-long structure built at KEK has been processed in 2004-2005, with an accumulated operation time with the RF turned on of ∼1000 hours. The RF breakdown rate of this structure at 65MV/m with 400nsec flat pulses was initially measured to be ∼once per 0.2×106 pulses, and decreased to ∼once per 0.7×106 pulses with the processing. This latter breakdown rate satisfies the stability requirement for use of such accelerator structures at the linear collider. The high-power performance of two more 60cm structures will be measured in 2005. These studies are expected to provide benchmark performance data of accelerator structures in both high-gradient and mediumgradient operations, such as those envisaged in applications to compact X-band accelerators.