Abstract: In this paper, the pattern variation of aperture antenna with different polarization versus the truncation size of reentry body(RB) mockup is analyzed with geometrical diffraction techniques(GDT). On the basis of analysis, truncation effect of RB mockup on antenna pattern is evaluated in elaboration with experiment, thus the minimum mockup sizes corresponding to different aperture polarization on which the antenna pattern measurement error due to truncation is neglectible is obtained.