Abstract: By using a XF Ⅱ type clinical diagnosis x ray dosimeter (a circular plate ionization chamber) as a monitor instrument in the calibration system with the standard ionization chamber of therapeutic level, the ratio of monitor value to standard value is observed for more than 2 years. After treatment the monitoring value with linear regression analysis, the linear correlation coefficient is better than 0.999 999 with standard deviations s fallen into (2.0~10.0)×10 4 R/min(1 R=2.58×10 -4 C/kg) respectively. The mean value of ratio for monitor value to standard value is 0.522 4 with a relative standard deviation s r=0.67% (for a single determination). The results indicate that it is stable and reliable by using the XF Ⅱ type clinical diagnosis X ray dosimeter as a monitor instrument.