Solanaceae plants are strong allelopathic in nature as they produce and release many chemical compounds into the environment. This study was carried out to investigate the phytotoxic effects of the seeds aqueous extract of thorn apple (Datua stramonium L.) on seed germination of sorghum (Sorghum bicolor [L.] Moench), millet (Pennisetum glaucum [L.] R. Br.), maize (Zea mays L.) and wheat (Triticum vulgare L.) using probit analysis. Laboratory experiments were carried out at the Faculty of Agricultural Sciences, University of Gezira, Sudan in season 2014/15. Ten concentrations (4.62, 9.26, 13.87, 18.51, 23.12, 27.74, 32.36, 36.98, 41.61 and 46.28 g/l) of the seeds aqueous extract of D. stramonium were prepared from the stock solution (100 g / l). A control with sterilized-distilled water was included for comparison. Treatments were arranged in completely randomized design with four replicates. The seeds were examined for inhibition (%) in germination at three days after initial germination. Data were transformed using Abbott’s formula and subjected to probit analysis (P  0.5). The results showed that the seeds aqueous extract of D. stramonium inhibited the seed germination of the tested cereal crops and there was direct positive relationship between concentration (g/l) and inhibition (%). The results also showed that the seeds of wheat (LC50 = 22.6 g/l) were most sensitive to the seeds aqueous extract of thorn apple followed by the seeds of sorghum (LC50 = 26.5 g/l) and maize (LC50 = 27.9 g/l). However, the extract was less toxic to the seeds of millet (LC50 = 32.2 g/l). It was concluded that that the aqueous extract of thorn apple (D. stramonium L.) was phytotoxic to the seed germination of the tested cereal crops.