Catharanthus roseus (L.) G. Don, the Madagascan periwinkle, is a source of clinically and commercially important antitumor bisindole alkaloids such as vincaleukoblastine (VLB) and leurocristine (VCR). Over 90 alkaloids have been isolated from this plant, which has been a prime focus of biosynthetic studies on monoterpenoid indole alkaloids. In this study, a new compound, leurosidine N'b-oxide (2), was isolated from the alkaloid fractions of C. roseus. Isolation was performed using silica gel column chromatography. The structure was elucidated through spectroscopic analyses including infrared (IR), ultraviolet (UV), proton nuclear magnetic resonance (¹H-NMR), carbon-13 nuclear magnetic resonance (¹³C-NMR), and mass spectrometry (MS). Confirmation of the structure was obtained by chemical reduction with 10% aqueous ferrous sulfate, which converted the compound to leurosidine. Biological activity evaluation showed that leurosidine N'b-oxide was cytotoxic to P-388 (ED₅₀ 2.7 μg/ml) and KB (ED₅₀ 0.26 μg/ml) cell lines in vitro.