Quasi-monochromatic parallel flash radiography system utilizing a plane-focus plasma x-ray tube in conjunction with an x-ray lens is described. The x-ray generator employs a high-voltage power supply, a low-impedance coaxial transmission line, a high-voltage condenser with a capacity of about 200 nF, a turbo-molecular pump, a krytron pulse generator as a trigger device, and a flash x-ray tube. The high-voltage main condenser is charged up to 50 kV by the power supply, and the electric charges in the condenser are discharged to the tube after triggering the cathode electrode. The flash x-rays are then produced. The x-ray tube is of a demountable triode that is connected to the turbo molecular pump with a pressure of approximately 1 mPa. As the electron flows from the cathode electrode are roughly converged to the target plane by the electric field in the tube, the weakly ionized plasma x-ray source, which consists of copper ions and electrons, forms by the target evaporating. Both the tube voltage and current displayed damped oscillations, and their peak values increased according to increases in the charging voltage. In the present work, the peak tube voltage was almost equal to the initial charging voltage of the main condenser, and the peak current was about 20 kA with a charging voltage of 50 kV. The dimension of x-ray source was almost equal to the target diameter of about 10 mm, and the x-ray pulse widths were less than 1 μs. When the charging voltage was increased, the plasma x-ray source formed, and the characteristic x-ray intensities of K-series lines substantially increased. The quasi-monochromatic x-rays from the plane-focus tube were formed into parallel beam by a polycapillary plate with a hole diameter and a thickness of 25 μm and 1.0 mm, respectively, and quasi-monochromatic radiography was performed by a film-less computed radiography system.