This paper presents the effect of decoupling capacitors in DC bias lines. This paper approaches the aspect of isolation between DC bias lines instead of power integrity. The effects depending on decoupling capacitors are studied in RF system by measuring the implemented wireless-personal-area-network (WPAN) system. The WPAN system is composed of a front-end-module by LTCC, a RF IC, and a baseband (FPGA) board. DC bias lines are tested and discussed from the point of view of both Z-parameter and S-parameter. The Z-parameter and S-parameter are measured by network analyzer and the performance of the system is measured. Decoupling capacitors in DC bias line are focused on when the WPAN system is performed in Tx mode. The parameter and the system performance depending DC bias lines with/without decoupling capacitors are measured. As a result, the correlations between system parameter and system performance depending on decoupling capacitors are analyzed and obtained. The correlation between parameter and system performance, the role of decoupling capacitors, and the required isolation in DC bias line by employing decoupling capacitors were discussed and reviewed in this paper. Analysis by using S-parameter was better than by using Z-parameter in order to estimate the performance of the RF system. Isolation between DC bias lines is ensured by using decoupling capacitors, and the performance is improved. Tx spectrum of the RF system was improved as the isolation between DC bias lines was increased above 15 dB.