The allelopathic influence of aqueous extracts of Eucalyptus camaldulensis on Cassia angustifolia Vahl., Datura stramonium L. and Hibiscus sabdariffa L. germination (%), germination stages and seedling growth were examined. For this goal, the extract was prepared from the shoot and roots of Eucalyptus camaldulensis firstly. The extract was diluted by adding distilled water to 0 (control), 25, 50, 75 and 100 percent. The effect of the four mentioned treatments, as well as distilled water as control were studied in four repetitions on seed germination using a completely randomized design. Results showed that there were significant differences among the measured characteristics for germination percentage and speed, length of root, length of shoot, length seedling and vigority index (p<1%) in all three species. Comparison of means indicated that increasing the concentration of allelopathic extract of Eucalyptus decreased seed germination and initial growth of seedlings in all three species significantly. The highest germination percentage and speed was belonging to Hibiscus sabdariffa (60% and 3.5) at zero concentration (control). Also, germination of Datura stramonium done only until concentration of 25 percent (20%) and at concentrations 50, 75 and 100 % it was zero. According to the results Datura stramonium had more sensibility than two other species related to allelopathic effect of Eucalyptus camaldulensis. Generally, it is recommended not to plant eucalyptus as a windbreak with medicinal species Cassia angustifolia, Datura stramonium and Hibiscus sabdariffa as much as possible in the study area.