An incubation experiment was carried out to investigate the effects of aqueous extracts of 30 aromatic plant species from 17 families on urea hydrolysis and nitrification in 3 soils. The results indicated that the aqueous extracts from 22 of the aromatic plants could inhibit urea hydrolysis, and 5 of the extracts showed better effects than urease inhibitor hydroquinone (HQ). The aqueous extracts from 30 of the aromatic plants could inhibit nitrification of NH₄⁺, and 11 of the extracts showed better effects than nitrification inhibitor dicyandiamide (DCD). Chamomila recutita belonging to Asteraceae and Citrus reticulate belonging to Rutaceae could effectively inhibit both urea hydrolysis and nitrification, and hence may be applied as synergist of urea. The results also indicated that the best inhibitory effects of C. recutita and C. reticulate on urea hydrolysis and nitrification appeared during 24-36 h and during 7-14 d of incubation, respectively, and their inhibitory effects increased with increasing their dosages.