The crude extracts of molting hormone which have been extracted from 8 selected plants were Achyranthes aspera, Cyathula prostrata, Gomphrena celosioides, Trianthema Portulacastrum, Sesuvium Portulacastrum, Morus alba, Vitex pinnate, Vitex canescens were tested for acute toxicity (96hr- LD50) with the giant tiger prawns, Penaeus monodon Fabricius and compared to B-ecdysone. The 96hr-LD50 of crude extracts were ranged from 0.347-3.7 mg/g body weight. The highest toxicity was the crude extract of G. celosioides and the lowest toxicity was the crude extract of M. alba. The results show that the crude extracts had shown the same characteristical death as B-ecdysone. From the results of toxicity test, we used the toxicity curve of crude extracts for setting the concentration which were used in the molting experiments of P. monodon. The prawns were injected both in the intermolt and premolt stages of the molt cycle. The show that the crude extracts and B-ecdysone could reduce the molt duration of P. monodon both in the intermolt and premolt stage by comparing to the control group. At the intermolt stage, the molt duration was shortened than the control by an average of 1.957 days (27.71%) at the lowest dosage (LD-O). The middle dosage (LD-25) shortened the molt duration by an average of days (26.53%) and the highest dosage (LD-50) shortened the molt duration by an average of 2.70 days (31.35%). The best result for shortening the molt duration was the crude extract of S. portulacastrum. At the premolt stage, the molt duration was shortened than the control by an average of 0.57 days (14.72%) at the lowest dosage (LD-0). The middle dosage (LD-25) shortened the molt duration by an average of 0.943 days (24.63%) and the highest dosage (LD-50) shortened the molt duration by an average of 1.17 days (30.18%). The best result for shortening the molt duration was the crude extract of M. alba. The increased dosages caused a greater shortening the molt duration than the corresponding treatments both in the intermolt and premolt stages but there were no statistical significance. The high dosage caused slightly greater mortality rate than the low dosage both in the premolt stage had a greater mortality rate than the intermolt stage. The crude extracts did not repress the subsequent growth increment after molt that it was adventageous for aquaculture purposes of P. monodon.