Effects of plant densities on basal stem vascular bundles and root bleeding sap of different density-tolerant maize cultivars were studied in order to provide theoretical basis for high-density planting.The density experiment was conducted in the field with a high density-tolerant maize cultivar (‘Zhengdan 958’) and a low density-tolerant maize cultivar (‘Liaodan 526’) as the experimental materials.The vascular bundle structure of the basal stem internodes was observed by microscope through the hand-sliced method.Moreover,the bleeding sap quantity and the main composition content were also analyzed.The results indicated that:(1)At the jointing stage,the structure of vascular bundle in ‘Zhengdan 958’ was relatively sensitive to changes of density,but there was no significant difference of vascular bundle in ‘Liaodan 526’ among densities.The bleeding sap quantity of ‘Liaodan 526’ decreased in larger extents than did ‘Zhengdan 958’ at the highest density and there was no regular changes in composition content of both cultivars with the density increased.(2)At the milking stage,densities significantly affected the structure of vascular bundle in ‘Liaodan 526’ but had no significant effects on it of ‘Zhengdan 958’.With the density increased,indicators of vascular bundle in ‘Liaodan 526’ decreased in evidence compared with the contrast.The bleeding sap quantity of ‘Zhengdan 958’ was larger than that of ‘Liaodan 526’ at all densities.Soluble sugar in root bleeding sap of both cultivars enhanced,especially in ‘Liaodan 526’.(3)The decreasing extents of yield in ‘Liaodan 526’ were larger than that in ‘Zhengdan 958’ when the density exceeded the contrast.Consequently,the response of high density-tolerant maize to changes of plant density was earlier and expressed a better adaptability than density-intolerant maize,which illuminated physiological mechanism for the difference of density tolerance.