Abstract: Many compounds from plants have been found to play an active role in inhibition and induction of GST activity. This study aims to evaluate the capability of five commonly used medicinal plants in Malaysia to inhibit the glutathione S-transferases (GST) activity in vitro. The ability of the plant extracts to inhibit GST activity was examined on rat liver cytosolic fraction and was analyzed using ultraviolet (UV) absorbance at 340 nm. When 1-chloro-2,4-dinitrobenzene (CDNB) was used as a substrate, tannic acid exerted inhibition with IC50 value of 6.18 μg mL-1. The methanol extracts of Orthosiphon stamineus and Cinnamomum iners demonstrated the highest inhibitory activity against GST activity showing IC50 values of 35.20±8.72 and 35.55±3.84 μg mL-1. It is noted that, at 250 μg mL-1 C. iners exhibited 1.3 more inhibition activity than tannic acid. The different parts of Croton argyratum plant were also evaluated on the GST inhibitory potential. Comparing the inhibition abilities of each part of Croton argyratum in decreasing order are leaves>roots> stems with IC50 values of 40.42, 57.88 and 143.80 μg mL-1, respectively. The data may suggest potential use of Orthosiphon stamineus and Cinnamomum iners as herbal medicines with GST inhibitory effect.