Low temperature cofired glass ceramic-Ag metal electrode (LTCC) systems were investigated in relation to Ag diffusion and microstructural development in the fabrication of high frequency couplers. Sintering temperature was in a range of 825 degrees C-975 degrees C. Ag diffusion was not observed below 875 degrees C but densification of the electrode was greatly improved. At higher temperature above 900 degrees C, Ag ions were diffused through glass phases containing Pb and alkali ions. Crystalline phases behaved as a barrier for the Ag diffusion. With increasing sintering temperature, glass infiltration to the electrode also occurred due to increased fluidity of the glass phase and the electrode line were severely deformed and damaged.