3-D FE analyses were conducted on lattice-type ground improvement by in-situ cement-mixing for liquefaction mitigation during earthquakes. By using an elasto-plastic model considering post-peak strain-softening properties in shear and tensile regions, seismic performance could be evaluated more rationally. Since tensile behavior of cement-treated sand was the key issue for the seismic performance of the lattice-type ground improvement, different types of laboratory tests and their numerical simulation were also performed. It was revealed that unconfined tension tests yield reasonable estimates of the actual tensile strength. Due to effects of shear failure below the loading strip, the tensile strength is underestimated in splitting tests. On the other hand, due to effects of stress re-distribution that is induced by the strain-softening behavior in the tensile region, the tensile strength is overestimated in bending tests.