We report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y(2)O(3)and SiO2 were used as the sintering aid for the pressureless sintering of the green tape. X-ray diffraction phase identification indicates that MgO is the main phase, while both Y2O3 and SiO2 sintering aids react with MgO to form MgY4Si3O13 as the second phase. Liquid phase sintering occurs in the temperature range from 1030 degrees C to similar to 1500 degrees C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650 degrees C for 2 hours, which was further increased to a fully dense structure using hot-isostatic-pressing(HIP) at 1650 degrees C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP'ed sample show that MgY(4)Si(3)O(13)is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP'ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.