Hypocotyls undergo different morphogenesis according to light and dark conditions. Cortical microtubules are reoriented in response to light to coordinate cell growth status. Kinesins are microtubule-based motor proteins that are mostly responsible for transporting organelles and vesicles, although some can also regulate microtubule organizations. However, whether kinesin can be involved in microtubule reorientation and hypocotyl elongation remains to be studied. Here, we demonstrated that ARMADILLO REPEAT KINESIN2 (ARK2) negatively regulated the hypocotyl elongation of Arabidopsis thaliana. The hypocotyl cells of the ark2 null allele were longer than those of the wild type and had relatively more transversely arranged cortical microtubules. Moreover, ARK2 co-localized with cortical microtubules and facilitated the light-induced reorientation of the cortical microtubule arrays. Interestingly, ARK2 protein is stable in the light and degraded through 26S proteasome pathway in the dark. Furthermore, we proved that ARK2 could interact with the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), which contributed to the down-regulation of ARK2 in darkness that may benefit hypocotyl growth in the dark.