Extraction and fractionation of the culm plus leaves of Chrysopogon serrulatus Trin. produce three major compounds, dibutyl phthalate (DBP), diphenylamine, 4,4′-dioctyl and simiarenol, which potentially act as allelochemicals or phytotoxic compounds. The effects of these compounds were examined using Leptochloa chinensis (L.) Nees as the bioassay species. The isolated compounds showed different degrees of inhibitory effects against L. chinensis. Of the three, DBP was the most potent and was able to inhibit L. chinensis germination by >70% at 500 mg L^–1, followed by simiarenol and diphenylamine, 4,4′-dioctyl with inhibition values ranging from 40 to 52%. Under aerobic conditions, DBP at 2.4 kg a.i. ha^–1 reduced the emergence and shoot fresh weight of L. chinensis by >50%, with negligible effect on root and shoot growth of aerobic rice seedlings, suggesting this as the most suitable rate and compound to control L. chinensis without injuring rice seedlings. At an application on the day of sowing rice seed, across DBP rates, reduction in root and shoot height of rice plants was evident when assessed 14 days after DBP treatment. However, rice plants become less susceptible with increasing growth stage. The findings suggest that DBP has potential to be developed as a pre-emergence, soil-applied natural herbicide for control of L. chinensis in aerobic rice system.