Abstract: Using ELISA analysis, the relation between endogenous hormone content and the thickening of fleshy roots of carrot (Daucus carrot L. var. sativa DC.) in summer seasons in plateau regions were analyzed. Normal fleshy roots of three carrot varieties (“Xinxiusanhong”, “Qishantouxinhong” and “Difangtouxinhong”) and bolting fleshy roots of “Qishangtouxinhong” were planted and investigated. Results showed that after seedling emergency for 35 days, growth rate of carrot fleshy roots of the three varieties initially increased and then decreased, sink activity continually dropped. Compared with normal fleshy roots, bolting fleshy roots had significantly lower growth rate and higher dry-to-fresh weight ratio (P<1%). Fleshy root dry weight was positively correlated with GA₃ content, but negatively correlated with IAA and IPA contents at 5%~1% significance level. IAA and GA₄ contents were also positively correlated with fleshy root dry-to-fresh weight ratio, whereas IAA, GA₄, ABA, GA₃, IPA and DHZR contents were positively correlated with fleshy root sink activity at 1% significance level. Endogenous hormone promoted carrot fleshy root thickening via controlling root sink activity in summer in plateau region.