The post-emergent herbicidal activity of eight monoterpenes, namely camphor, (R)-carvone, 1,8-cineole, cuminaldehyde, (S)-fenchone, geraniol, (S)-limonene and (R)-linalool was evaluated against Portulaca oleracea (L.) under glasshouse conditions at 1 and 2% concentration. Results were taken after 5 days of foliar applications. The effect of monoterpenes on chlorophyll contents and total phenolic compounds was also examined at 0.5 and 1% concentrations. All the tested monoterpenes reduced fresh, dry weights and reduced shoot growth of the weed. At 1%, cuminaldehyde caused the highest reduction of 97.4, 92.0 and 74.7% on fresh and dry weights, and shoot length, respectively and geraniol caused 94.1, 90.0 and 72.4 % reductions on fresh, dry weights, and shoot length, respectively. Moreover, cuminaldehyde and geraniol caused complete burning of the plant and complete reduction 100% of fresh weight, dry weight and shoot length at 2%. In contrary, camphor caused the lowest reduction of fresh, dry weight, and shoot length at both concentrations whereas the reduction reached 32.6, 34.6 and 14.8% at 1%, and 63.1, 62.8 and 33.6% at 2%, respectively. The tested monoterpenes showed significant reduction of both chlorophyll a and b contents and phenolic compounds at concentration of 0.5 and 1%, this indicated that monoterpenes may cause adverse effect on photosynthesis and weed metabolism.