Insight into the phase-dependent electron transfer kinetics and electrocatalytic activity of metal oxide nanostructures is important in the rational design of functional nanostructures for realizing high-performance electrochemical sensors. This study focuses on elucidating the effect of the crystalline phase on the electron transfer kinetics and electrocatalytic activity of iron(III) oxide. The α -FeOOH, γ -Fe 2 O 3 , and α -Fe 2 O 3 nanorods were designed by using a simple chemical method and calcining process. The phase-dependent difference in the electron transfer kinetics and electrocatalytic activity toward the sensitive response of chloramphenicol (CAP) is observed by the transformation from α -FeOOH to γ -Fe 2 O 3 , and from α -FeOOH to α -Fe 2 O 3 nanorods. We found that the oxygen vacancies formed in phase transformation from α -FeOOH to α -Fe 2 O 3 is a key factor in promoting the electrochemical reduction of chloramphenicol. The α -Fe 2 O 3 nanorods-based electrochemical sensors showed a linear response in the CAP concentration range from 0.1 to 75 μ M with a limit of detection of 60 nM and an electrochemical sensitivity of 2.86 μ A μ M −1 cm −2 . This work further provides valuable physical insight into the phase-dependent electron transfer kinetics and electrocatalytic activity of metal oxide nanostructures for the rational design of sensing interface.