Melilite-type crystals, including Ca2Ga2SiO7, Ca2MgSi2O7, and CaNdGa3O7, were successfully grown by the Czochralski technique. Thermal properties were investigated and full matrices of electromechanical constants of these melilite crystals were evaluated by impedance method, with d14 being on the order of 5.3-9.3 pC/N. The relationship between microstructure and electromechanical properties of the three kinds of crystals was established, in order to explore the piezoelectric origin and further optimize the piezoelectric properties. The distortion of eight-fold antiprisms and the distortion/rotation of tetrahedrons were thought to contribute to the piezoelectric d14 and d36, respectively. In addition, the layered structure leads to strong anisotropic behavior, accounts for the high resistivity along Z direction in Ca2MgSi2O7, while the disordered structure accounts for the degraded resistivity in CaNdGa3O7. The properties at elevated temperature were investigated, where the elastic constants show high thermal stability over the range of 25-500 C, with minimal variation of 6%.