In this paper described is the absorbed dose deternination of three types X(g) knives at isocenter point in different radiation fields by using a 160-mm diameter PMMA spherical phantom and a 0.015cm3 PTW 31006 type pinpoint ionization chamber which can be inserted into the center of the phantom. The measured data were compared with those measured by a semiconductor detector (with sensitive volume of 1mm?mm?.5mm). A comparative study on the applicability of the two detectors was carried out

and a method suitable for the absorbed dose determination of X(g) knife was put forward.