Grape seed extract (GSE) is one of Japan's natural food additives, and is recommended for use as an antioxidant. The component of interest is polyphenol, mainly proanthocyanidins, which are condensed tannins. A vanillin-hydrochloric acid (HCl) assay is specific for flavan-3-ol. The sensitivity of GSE components to the vanillin-HCl assay differed according to the chemical structure. We applied this assay to the determination of proanthocyanidin content in GSEs, health foods containing GSEs and grape seed oils. The concentration of proanthocyanidins in GSEs and health foods was overestimated in the vanillin-HCl assay when (+)-catechin was used as a standard. However, the concentration of proanthocyanidins in health foods with high protein or a high colored substance content was underestimated. No proanthocyanidins were detected in grape seed oil. Furthermore, we examined the content of low molecular weight compounds in GSE by reversed-phase HPLC. The percentage of low molecular weight compounds, including gallic acid, (+)-catechin, (-)-epicatechin, procyanidins B1, B2 and C was estimated to be 5.5-12.2%(w/w).