In this paper, the effect of underpass structure on quality factor and breakdown voltage ofoctagonal inductors which were fabricated with 90 nm complementary metal-oxide-semiconductor (CMOS)technology for radio frequency integrated circuit (RFIC) was studied. It was found that quality factor andbreakdown voltage of inductors with more than one metal layer for underpass showed improvedproperties compared to those with one metal layer. However, little change of quality factor andbreakdown voltage was observed between the inductors with two and more than two metal layers forunderpass. Therefore, underpasses with two metal layers are promising for RFIC designs of the octagonalinductors in 90 nm CMOS technology.