A high-performance liquid chromatography method employing diode array detection was developed to determine levels of the major catechins, proanthocyanidin (procyanidin B2), caffeine, thymoquinone and carvacrol and its isomer, thymol, which are present in different natural complex matrices found in commercial products of Camellia sinensis L. and/or Nigella sativa L. Reversed-phase separation was performed on a C18 column by using gradient elution by varying the proportions of solvent A (distilled water containing 0.05% orthophosphoric acid) and solvent B (acetonitrile), with a flow rate of 1.5 mL/min and duration of 31 min. Excellent linearity was observed for all standard calibration curves, and correlation coefficients were above 0.9996. The developed method is efficient, with high reproducibility and sensitivity, and is ideally suited for rapid and routine analysis of principal components in these promising medicinal plants.