Abstract: Using transgenic Bt cotton “Guokang 22” and conventional parental cotton “Simian No. 3” as tested varieties, the effect of introducing Bt gene on physiological indices of vascular bundle sap of cotton leaf and reproduction of Bemisia tabaci (Gennadius) was studied. The results show that transgenic Bt cotton “Guokang 22” contains more soluble sugar in vascular bundle sap both at seedling and flowering stages than the conventional parental cotton “Simian No.3”. Tannin content in vascular bundle sap of “Guokang 22” is lower than that of “Simian No.3” at seedling stage, but not significantly different at flowering stage. Although no significant difference exists in total free amino acid content between two varieties, some amino acids (including glutamic acid, proline and alanine) that are closely related to B. tabaci metabolism are significantly higher in “Guokang 22” than in “Simian No.3” at flowering stage. No gossypol is detected from the sap at both seeding and flowering stages. The intrinsic rates of increase (r_m) of B-type B. tabaci feeding on the leaves of trangenic Bt cotton “Guokang 22” at seeding and flowering stages are respectively 13.7% and 20.2% higher than those feeding on conventional parental cotton “Simian No.3”. This indicates that the introduction of foreign Bt gene produces profound effect on the syntheses of soluble sugars, free amino acids and anti-biosies, and thus on the population of B. tabaci.