The x-ray flat panel detector (FPD) is a key component of the next generation x-ray imaging systems which promote digitization of x-ray images. By developing FPD applicable to both fluoroscopy and radiography, it is expected that x-ray diagnostic systems will change dramatically not only in terms of performance, but also in shape and form. The purpose of this research is to develop a selenium-based FPD applicable to both radiography and fluoroscopy. This report presents the results of the work on new technology which can be applied to create clinically-useful view size detector. The prototype detector adopts 1000 micron thick selenium as the photoconductor, 23 cm square of field of view, 150 X 150 micron pixel pitch, 1536 X 1536 pixel number, and is capable of capturing images at up to 30 frames/second. The features of this prototype detector are: simple-structure selenium suitable for real-time readout, and a high voltage protection structure for the TFT array which acts at x-ray overexposure. To realize large field of view, the field uniformity performance of the selenium and TFT array has been improved, and noise from the TFT array has been minimized. In this paper, new physical performance related to MTF, input- and-output characteristics, image lag, blooming, etc., are discussed.