The response of a digital computed radiography system to the megavoltage therapeutic radiation beams was invested. A narrow slit of radiation beam was used to test the line spread function of the system. The effects of various facts such as cassette, beam energy, radiation dose, scanning orientation and timing on the line spread function were investigated. The calibration curves were established to calibrate the image intensity to the megavoltage radiation dose. The calibration curves were applied to measure the beam profiles of the radiation fields with various wedges.