Iron is a vital micronutrient for growth of bloom-forming Microcystis aeruginosa and competition with other algae, and its availability is affected by humic acid. The effect of iron and humic acid on growth and competition between M. aeruginosa and Scenedesmus obliquus was assessed. The results showed the growth of M. aeruginosa and S. obliquus in mono-cultures was inhibited by humic acid at low iron concentrations (0.01 mg/L); the maximum inhibition ratios were 67.84% and 38.31%, respectively. The inhibition of humic acid on the two species was significantly alleviated when iron concentrations were 1.00 mg/L, with the maximum inhibition rate reduced to 5.82% for M. aeruginosa and to 23.06% for S. obliquus. S. obliquus was the dominant species in mixed cultures, and the mutual inhibition between M. aeruginosa and S. obliquus at low iron concentration was greater than that at high iron concentration. The inhibition of S. obliquus on M. aeruginosa was reduced at low iron concentrations; it increased at high iron concentrations, as concentrations of humic acid rose.