Extraction techniques using Supercritical Fluid Extraction (SFE) and Liquid Solvent Extraction (LSE) were evaluated for the extraction of Ent-11 alpha-hydroxy-15-oxo-kaur-16-en-19-oic-acid (5 F), the antitumor diterpenoid from Pteris semipinnata L. The extracts were analyzed by high performance liquid chromatography (HPLC). SFE experiments showed that many factors had a great impact on the yield and purity of the diterpenoid, such as extraction temperature, pressure, fluid flow rate, extraction time, and modifier. For the SFE process, the optimum operation conditions were as follows: extraction temperature of 328.15 K, extraction pressure of 30 MPa, supercritical CO2 flow rate of 160 kg/h, extraction time of 4 h, and 10% ethanol as the modifier. Under such a condition, the diterpenoid was almost completely extracted from the material and the yield was approximately 0.504 g/kg dry herb by HPLC analysis. The yield was approximately 3 fold higher than that by liquid solvent extraction. The purity of 5F was 5.148 g/kg dried extract with SFE, it was about 9 fold higher than that by LSE. Mass spectrum data indicated there were two correlative compounds, 5F and its derivative with glycose, in both the extracts, and the ratio of the signal strength of 5F and its derivative was about 3:1 in the SFE extract while that ratio was 1:3 in the LSE extract. The results demonstrated that the supercritical fluid extraction was selective, highly efficient, and with less consumption of organic solvents.