Quantitative analysis of trans-10-hydroxy-2-decenoic acid (10-HDA) in pure royal jelly creams and dietary supplements of royal jelly available in the United States was carried out via reversed phase high performance liquid chromatography (HPLC). The target compound, 10-HDA, in samples and an internal standard, methyl 4-hydroxybenzoate (MHB), were separated using a Zorbax Eclipse XDB-C18 column (150 × 4.6 mm) with a mobile phase composed of methanol and an aqueous solution of phosphoric acid at 25°C. The flow rate was 1.0 mL/min and the UV detection was performed at 215 nm. The average recovery rate of 10-HDA was 97.4 – 100.4% with the relative standard deviation (RSD) of 2.4 – 3.4% over the concentrations ranging from 10 to 80 μg/mL. The limit of detection (LOD) and limit of quantification (LOQ) were found to be about 0.05 and 0.25 μg/mL, respectively. Our results show that the concentration of 10-HDA lies between 1.85 and 2.18% for pure royal jelly creams and between 0.43 and 6.28% for royal jelly supplements.