Abstract: Root morphology and distribution have an important influence on the absorption of water and nutrients in soil. Increased resistance to planting density is one of the major traits in modern maize breeding, but little is known about the role of the plastic response of root system in the resistance to density in maize. In this paper, two old non-density resistant hybrids (Zhongdan2 and Danyu13) and two modern density-resistant hybrids (Xianyu335 and Zhengdan958) which had or have been widely used in different eras were adopted to investigate the root growth and its response to increasing planting density under both field condition and hydroponics. Results showed that the size of root system of the modern density-resistant cultivars is smaller than that of the old non-resistant cultivars and the difference mainly happened in 0-40 cm soil layer. Along with the increase of planting density, the root becomes smaller and thinner significantly. High planting density only reduced root length in the 0-20 cm soil layer, without significant effect on deeper roots. The response of root length to planting density was weaker in Xianyu335 and Zhongdan2, but stronger in Zhengdan958 and Danyu13. It is concluded that, to adapt to high planting density, Xianyu335 relies mainly on its small root size while Zhengdan958 depends on both small root size and high root plasticity.