In this paper, we study how coseismic deformations calculated in 1066 Earth models are affected by how the models treat Earth discontinuities. From the results of applying models 1066A (continuous) and 1066B (discontinuous), we find that the difference in Love numbers of strike-slip and horizontal tensile sources are bigger than dip-slip and vertical tensile sources. Taken collectively, discontinuities have major effects on Green’s functions of four independent sources. For the near-field coseismic deformations of the 2013 Okhotsk earthquake (Mw 8.3), the overall differences between theoretical calculations in vertical displacement, geoid, and gravity changes caused by discontinuities are 10.52 percent, 9.07 percent and 6.19 percent, with RMS errors of 0.624 mm, 0.029 mm, and 0.063 μGal, respectively. The difference in far-field displacements is small, compared with GPS data, and we can neglect this effect. For the shallow earthquake, 2011 Tohoku-Oki earthquake (Mw 9.0), the differences in near-field displacements are 0.030 m (N-S), 0.093 m (E-W), and 0.025 m (up-down) in our study area with the ARIA slip model, which gives results closer to GPS data than those from the USGS model. The difference in vertical displacements and gravity changes on the Earth’s surface caused by discontinuities are larger than 10 percent. The difference in the theoretical gravity changes at spatially fixed points truncated to degrees 60, as required by GRACE data, is 0.0016 μGal and the discrepancy is 11 percent, with the theoretical spatial gravity changes from 1066B closer to observations than from 1066A. The results show that an Earth model with discontinuities in the medium has a large effect on the calculated coseismic deformations.