Simultaneous switching noise (SSN) and electromagnetic radiation were intensively examined experimentally by using different test LSIs and packages. Two types of test LSIs have been designed and fabricated in 0.25 um CMOS process. One was with on-chip decoupling capacitance (ODC), and the other was without ODC. Then, two types of packages; quad flat package (QFP) and chip scale package (CSP) were used for housing them. First, SSN and radiated emission for the combination of two types of chips and two types of packages were measured by exciting output circuit and core logic circuit independently. QFP with larger package inductance showed larger power/ground fluctuation than CSP. On the other hand, QFP showed smaller emission properties than CSP. It is because package inductance worked as a filtering component. For both packages, every harmonics was reduced for the core circuit operation due to ODC, while it has been found that only even-order harmonics were dramatically decreased for the output buffer circuit operation.