Abstract: LC-MS/MS was applied to the determination of fipronil and its 3 metabolites in eggs. The egg sample was extracted with acetonitrile. After centrifugation, the supernatant was evaporated to near dryness by N₂-blowing. The residue was taken up with water and then purified with HLB solid phase extraction column. Shim-pack XR-ODS C₁₈ chromatographic column was used as stationary phase, and the mixture of 4 mmol·L^-1 ammonium acetate solution and methanol mixed in different ratios was used as mobile phase in gradient elution. ESI^- and multi-reactions monitor mode were adopted in MS/MS. Linear relationships between values of peak area and mass fraction of fipronil and its 3 metabolites were kept in the same range of 1-50 μg·kg^-1, with same detection limit (3S/N) of 0.3 μg·kg^-1. Tests for recovery were made by standard addition method at the concentration levels of 1, 5, 20 μg·kg^-1, giving values of recovery and RSD's (n=6) in the ranges of 89.9%-96.1% and 1.6%-3.3% respectively.