Abstract: AIMTo establish an accurate and reliable method for quantitative analysis of the diterpenoids in Pteris semipinnata L. METHODSA quadruple mass spectrometer coupled with atmospheric pressure chemical ionization interface was employed as a detector for HPLC. As to MS detector, selective ion monitoring (SIM) scan mode was used. For ent-11α-hydroxy-15-oxo-kaur-16-en-19-olic acid (5F) and ent-11α-hydroxy-15-oxo-kaur-16(R) methyl-19-olic acid (4F), the majority of the diterpenoids in Pteris semipinnata L, the ［M-H］^-1 ion were observed, and the ［M-H₂O-H］^-1 ion could be observed from the collision-induced dissociation spectua. ［M-H］^-1 was selected as the SIM ion in quantification, the mobile phase and the MS conditions were optimized. The mobile phase of HPLC was 30% CH₃CN-70% 2 mmol·L^-1 NH₄Ac, analytical column was Diamonsil ODS (4.6 mm×150 mm), flow rate 1.0 mL·min^-1, inject volume 5 μL. The area of ion flow peak were used for quantitative determination. As an example of its application, this method was used to determine the content of 5F as an antitumor diterpenoid in Pteris semipinnata L. RESULTSThe content of 5F accounted 1.18 mg·g^-1 in Pteris semipinnata L sample. For 5F, RT is about 4.3 min, the standard curve showed good linearity over the range of 0.05～2.5 μg, γ=0.9998 (N=5); the recovery was 97.8% (N=5); the limit of detection was 0.4 ng (inject 5 μL ). CONCLUSIONThis method is highly sensitive,accurate and fast, which can be applied to study the antitumor drug of diterpenoids in Pteris semipinnata L and to establish the raw herb standard.