The new LTCC material has been developed by crystallization of glass compositions in the firing process so that the major amount of glass becomes cordierite. The new material has CTE (coefficient of thermal expansion) of 3.4E-6/degree Celsius and dielectric constant of 5.6. The CTE of the new LTCC material is very close to that of silicon, which realizes high mounting reliability of a silicon device. We have measured the maximum principal stress in a silicon device mounted on test packages, using a piezoelectric element built-in the device. In the case of our new LTCC package, the stress in the device was around 0 Mp, while a Build-Up package (CTE: 17.0 E-6/degree Celsius) has shown the stress about 80 MPa. Our new LTCC material can be co-fired with copper conductor, which realized high effective conductivity and low transmission loss. The new LTCC material has been confirmed to be a promising solution of packages for Low-k and Ultra Low-k devices.