High performance low temperature co-fired ceramic (LTCC) dielectrics is highly desired for next generation information technology. The rational design is a key issue for the development of new LTCC materials. In comparison to the design of conventional electroceramics, more attention should be paid on the formation process of the material structure for that of LTCC, in addition to the physical properties, due to the special requirement in fabrication processing. In this paper, sintering mechanism of three types of LTCC materials, i.e., glass-ceramics, glass ceramic composite, and glass bonded ceramics, as well as important factors of their dielectric properties are discussed and summarized, and the design strategies for LTCC dielectrics, based on new matrix materials with much lower sintering temperature or higher quality, are proposed. As an example for rational design, oxyfluoride glass-ceramic based dielectrics, a new class of LTCC materials with low r, is analyzed.