A method for the determination of pesticides by gas chromatograph/mass spectrometry (GC/MS) using internal standards was developed. A GC/MS type HP-5980 equipped with a SIM detector and a 0.25 mm i.d.×30 m glass capillary column cross-linked with 0.1 μm of 100% dimethyl polysiloxane (DB-1) was mainly used. The internal standards used as retention index standards of GC were n-alkylbis (trifluoromethyl) phosphine sulfides [(CF₃)₂-PS-(CH₂)_nCH₃]. The mass spectrum of each compound (n=9, 11, 13, 15, 17 and 19) showed strong fragment ion peaks at m/z M⁺-69 and M⁺-101, and these peaks were used for the monitoring of fragment ions. The results of experiments with 32 pesticides showed the coefficents of variations (CV%) of pesticides of 2.1-16.6% with internal standards and 4.2-29.2% without internal standards at 0.2-0.5 ng of pesticides. The values for 2.0 ng of pesticides were 0.5-9.1%, and 5.3-14.0% with and without internal standards, respectively. This method using internal standards provides good reproducibility in the analysis of pesticides.