Abstract: Two alkaloids (Ⅰ and Ⅱ) were isolated from the folk medicine——Crotarlaia sessiliflora L. Alkaloid I was found to be monocrotalline, while alkaloid Ⅱ is a new pyrrolizidine alkaloid, from which trans-trans platynecic acid (trans, 2 S, 3 R) and retronecine were obtained after hydrolysis. Thus, it is proposed to have the structural formula (C). Alkaloid Ⅰ showed inhibitory activity against several rodent tumors such as sarcoma 180, leukemia 615 and walker carcinoma 256. Eight derivatives of monocrotalline were prepared in an attempt to reduce its hepatic toxicity. Three (1, 7, 8)of them were shown to possess antitumor activity against walker carcinoma 256 in rats, but the toxicity showed in the preclinical studies kept them from entering clinical study.