Method development and validation of decursin for the standardization of Angelica gigas Nakai as a functional ingredient and health food were accomplished. The quantitative determination method of decursin as a marker compound of aerial parts of Angelica gigas Nakai extract (AAGE) was optimized by HPLC analysis using a C18 column (3×150 mm, 3 μm) with 0.1% TFA in water and acetonitrile as the mobile phase at a flow rate of 0.5 mL/ min and detection wavelength of 330 nm. The HPLC/PDA method was applied successfully to quantification of the marker compound in AAGE after validation of the method with linearity, accuracy, and precision. The method showed high linearity in the calibration curve at a coefficient of correlation (R2) of 0.9994 and the limit of detection and limit of quantitation were 0.011 μg/mL and 0.033 μg/mL, respectively. Relative standard deviation (RSD) values of data from intra- and inter-day precision were less than 1.10% and 1.13%, respectively. Recovery of decursin at 0.5, 1, 5 and 10 μg/mL were 92.38 ~ 104.11%. These results suggest that the developed HPLC method is very useful for the determination of marker compound in AAGE to develop a health functional material.