This paper reports a frequency trimming method of a quad-mass MEMS gyroscope using focus ion beam (FIB). A quad-mass gyroscope (QMG) using the standard SOI MEMS process was developed. The relationship between trimming geometry and resonant frequency was estimated by finite element analysis (FEA) and validated by experiment. The resonant frequency mismatch was initially about 2791 ppm. The proposed trimming method enables to reduce the frequency mismatch as small as 3 ppm with a small change of Q factor.