A key step in Image Guided Surgery (IGS) is the accurate preoperative registration of CT/MRI. Landmark-based registration performs much better in speed, stability and interaction than other registration methods, and the registration accuracy, which directly affects image-to-target registration in surgery, mainly depends on manually localization. In this thesis, we present an experimental technique for estimating the fiducial localization error (FLE_I) and analyzing effecting factors on FLE_I. Our study shows that theoretical localization accuracy is 0.57±0.18mm while clinic accuracy is 0.66±0.33mm (CT) and 0.98±0.38mm (MRI), from which we conclude that fiducial marker based registration method is stable and accurate enough which is an appropriate choice for an image guided surgery system.