The effects of sintering temperature on microstructural evolution and electrical characteristics of screen printed Cu patterns were observed. A commercial conducting paste containing Cu nanoparticles was screen printed onto a sodalime wafer sintered under a sintering temperature range of 300 degrees C to 450 degrees C. A network analyzer and Cascade's probe system in the frequency range of 10 MHz to 20 GHz were employed to measure the S-parameters of the sintered Cu conducting patterns. From the measured S-parameters, the insertion losses in high frequencies decreased with increasing sintering temperature due to the formation of an interparticle necking after heat treatment at high temperatures. However, oxidation of Cu nanoparticles during the sintering deteriorated the RF performance of the circuits, resulting large deviation of the S-parameters from the simulated curves.