This paper describes the noise properties of a Se-based flat-panel X-ray detector (FPD) with 128-channel CMOS readout integrated circuits (ICs). The detector used in this study is a 23 cm X 23 cm direct detection FPD utilizing a thick a-Se film. The TFT array format is 1536 X 1536 pixels with a pixel pitch of 150 micrometers . The key component of this FPD is a newly developed CMOS readout IC, which integrates 128 readout channels. Each channel consists of a charge-sensitive amplifier (CSA), a low-pass filter (LPF), a correlated double sampling circuit (CDS), and an analog sample-hold circuit. To evaluate the noise properties, theoretical noise models, including a precise expression for the CDS, were constructed and compared with the measured results, which were obtained by analyzing the noise power spectra (NPS) of the dark images taken without X-ray exposure. In addition, the detective quantum efficiency (DQE) was measured at several fluoroscopic dose ranges to demonstrate the overall signal-to-noise performance. As a result, a total readout electronic noise of 2,100 e- rms was obtained, in which thermal noise and 1/f noise of the amplifier was dominant. The DQE(0) measured was about 0.5 at 1 (mu) R.