Abstract: In this paper,the double-level frontal zones under south branch jet in spring over south of China are investigated based on the dense radiosonde resports.The results calculated from the frontagenetical formula show that the lower-level frontal zone is mainly due to the air flow coavergence,whilc the upper-level frontal zone is primarily caused by the different distribution of vertical motion,i.e. ∂ω/∂y<0.