Almost every electronic equipment must be connected to power grid. Because of the complexity of power lines, the reduction of conductive electromagnetic interference (C-EMI) plays an important role in precise measurements. Electromagnetic interference (EMI) is a major problem for electronic equipment, especially for those sophisticated instruments using in TLS (Taiwan Light Source). Therefore, a lot of efforts have been made to minimize the electromagnetic noise between instruments and power grid [1]. In this paper, a line impedance stabilization network (LISN) was built up to get the power spectrum of various instruments from power grid. After measuring some commercial power supplies, it is found that some of these power supplies induce effectively C-EMI into power grid, even though a passive filter is bound in power line. These noises may influence numerous equipments in a local area near the sources. Therefore, how to choose a suitable filter to block off noise is a decisive factor to reduce the effects of C-EMI.