dl-Tetrahydropalmatine (dl-THP), an alkaloid occurring in plant, has been intensively studied due to its pharmacological actions. In this report, dl-THP from rhizome of Corydalis yanhusuo W.T. Wang was extracted using supercritical fluid extraction (SFE). Determinations of the extracts were performed by high-performance liquid chromatography. An orthogonal array design (OAD), OA(9)(3(4)), was employed for optimization of supercritical fluid extraction of parameters of the compound. Four factors, namely various modifiers, the dynamic extraction time, temperature and pressure of the supercritical fluid, were studied and optimized by a three-level OAD. The effects of the parameters on the yield of dl-THP were studied using analysis of variance (ANOVA). Temperature had significant effect on the yield of dl-THP, while the other three factors, i.e. various modifiers, dynamic extraction time and pressure, were not identified as significant factors under the selected conditions, based on ANOVA. The result shown that the optimization for the extraction condition of dl-THP was 1,2-propanediol used as modifier of supercritical CO2, extraction time 60 min, temperature 70 degrees C and pressure 200 bar, and that higher yield (1.324 mg/g) was obtained than that obtained by the other extraction condition. (C) 2008 Elsevier B.V. All rights reserved.