This paper describes a newly developed method using gas chromatography-selected ion monitoring (GC-SIM) for the simultaneous determination of diester-diterpene type Aconitum alkaloids and their hydrolysis products in human serum. When the alkaloids were converted into their TMS derivatives, they produced a well defined peak on selected ion recording (SIR). Good linear response over the range of 100 pg to 7.5 ng was demonstrated for each alkaloid. When the alkaloids were spiked into human serum, good quantitative recovery was observed.