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We report the relationship between the $Gaia$--VLBI position differences andthe magnitudes of source structure effects in VLBI observations. Because the$Gaia$--VLBI position differences are statistically significant for aconsiderable number of common sources, we attempt to discuss and explain theseposition differences based on VLBI observations and available source images atcm-wavelengths. Based on the derived closure amplitude root-mean-square(CARMS), which quantifies the magnitudes of source structure effects in theVLBI observations used for building the third realization of the InternationalCelestial Reference Frame, the arc lengths and normalized arc lengths of theposition differences are examined in detail. The radio jet directions and thedirections of the $Gaia$--VLBI position differences are investigated for asmall sample of sources. Both the arc lengths and normalized arc lengths of the$Gaia$ and VLBI positions are found to increase with the CARMS values. Themajority of the sources with statistically significant position differences areassociated with the sources having extended structure. Radio source structureis the one of the major factors of these position differences, and it can bethe dominate factor for a number of sources. The vectors of the $Gaia$ and VLBIposition differences are parallel to the radio-jet directions, which isconfirmed with stronger evidence.

Evidence of the $Gaia$--VLBI position differences being related to radio source structure