Abstract: AimTo study the antitumor peptide components in the stems and leaves of mistletoe (Viscum coloratum (Kom.) Nakai), the primary structure of the novel peptide was elucidated. Methods Cation exchange, gel filtration and HPLC were employed for isolation and purification. Matrix Assisted Laser Desorption Ionization-Time of Flight-Mass Spectrometry was used to determine the mass. The complete amino acid sequence of the novel peptide was obtained by Edman degradation combined with enzyme digestion. The antitumor activity of the peptide in vitro was studied with MTT method. Results The primary stucture of the peptide named as viscotoxin B2 is KSCCKNTTGRNIYNTCRFAGGSR ERCAKLSGCKIISASTCPSDYPK. The IC₅₀ value of viscotoxin B2 on the Rat Osteoblast-like Sarcoma 17/2.8 cells in vitro is 1.6 mg·L^-1. ConclusionViscotoxin B2 in V.coloratum, which has high similarity with viscotoxins from V.album, showed antitumor activity.