A quinolizidine alkaloid (sparteine) was isolated from t he ethanol extracts of the seed of Thermopsis lanceolata.The structure of sp arteine was elucidated on the basis of physical constants and spectroscopic evid ences.Bioassay showed that the conidia germination of the four plant pathogenic fungi tested were inhibited by the alkaloid.The antifungal activity of the alkal oid against the conidia of two fungi was strong,their EC_50 were 22 μ g/mL of Sphaeropsis sapinea and 26 μg/mL of Botryis cinerea respective ly.The antifungal activities of 75% Daconial’s EC_50 were 58 μg/mL o f F.oxysporum, 19 μg/mL of C. oxysporum,0.8 μg/mL of S.sapinea and 4 μg/mL of B. cinerea respectively under the same conditions of the bioassay.The mycelia growth of the seven plant pathogenic fungi test were also inhibited by the alkaloid,but its inhibitory activities on the seven mycilia was weaker than those on the conidia mentioned above.