Cultivar mixtures increase crop diversification and grain yield stability.  Achieving high grain yield and nitrogen use efficiency (NUE) with environmentally friendly practices is a major challenge, but it is currently unclear whether maize cultivar mixtures can improve NUE.  A two-year field experiment was conducted using two maize cultivars with different roots angles and leaf angles planted in monoculture or in mixtures under four nitrogen levels N0 (0 kg N ha^–1), N140 (140 kg N ha^–1), N280 (280 kg N ha^–1) and N340 (340 kg N ha^–1).  Cultivar mixtures significantly increased light interception of the middle canopy, dry matter accumulation and total root length under N0, N140, and N280 conditions.  Light interception of the middle canopy was positively related to dry matter accumulation and thus increased grain yield.  In addition, light interception of the whole canopy was positively related to total lateral root length, while the greater total lateral root length of outer nodal roots significantly improved nitrogen accumulation and NUE.  Thus, cultivar mixtures promoted an optimal canopy structure and good root growth, thereby improving grain yield and NUE.  These findings deepen our understanding of the facilitating effect of canopy structure and root traits of cultivar mixtures on the combined promotion of grain yield and NUE.