Various microstrips with a ferromagnetic core were designed and fabricated on a silicon substrate. The core was formed by a 0.5-?m-thick Ni78Fe22 film, patterned into rectangular prisms. Measurement results for attenuation constant versus frequency show a peak value of ? 50?dB/cm around 10?GHz. Electromagnetic simulations show that the attenuation observed is due to the energy exchange between the quasi-TEM mode of the microstrip and magnetostatic surface modes excited in the direction perpendicular to the signal line.