Two sorghum varieties (Liaoza 10 and Liaoza 19) were used as the plant materials and exogenous auxin (IAA) was sprayed on the plant leaves at the threeleaf stage with three concentrations of exogenous IAA (0, 100, 200 mg·L^-1, respectively). Effects of exogenous IAA application on endogenous hormone contents and tillering in sorghum were determined. Results showed that exogenous IAA application significantly affected the endogenous hormone contents of sorghum. Exogenous IAA application substantially increased endogenous IAA and gibberellic acid 3 (GA₃) contents but decreased zeatin riboside (ZR) and abscisic acid (ABA) contents in sorghum 3 days after spraying IAA. Endogenous IAA, GA₃ and ABA contents obviously declined but ZR content increased 6 days after the treatment of IAA. The exogenous IAA enhanced the probability of tillering at the first and fourth tillering nodes and decreased the probability of tillering at the second tillering node in sorghum. The exogenous IAA application inhibited the occurrence of multitillers by increasing the probability of a single tiller per plant. Meanwhile, the exogenous IAA reduced the tillering incidence and ineffective tillering rate. Exogenous IAA application increased the sorghum yield by promoting the main agronomic characteristics including panicle length, 1000grain weight, and grain weight per panicle.