HAPs, similar to Heme Activator Proteins (HAP) or nuclear factor-Y (NF-Y) in yeast and animals, play versatile roles in plant growth, development, and responses to environmental cues. HAP3b in Arabidopsis is a member in the HAP3 gene family and is involved in regulating flowering time through the long-day photoperiod pathway (Cai et al., 2007, Plant Physiol 145: 98-105). In this study, we report that overexpression of HAP3b enhances primary root elongation. Detailed analysis showed that HAP3b-overexpression did not affect the length of the root elongation zone and the cell length profiles in the elongation zone. Kinematic analysis indicated that root cells in HAP3b-overexpressors elongate faster than the cells in wild-type roots. Using GUS as a reporter gene, we showed that HAP3b is specifically expressed in the tip region of the root, where cell division and elongation occur. Our results provide evidence to support a role of HAP3b in regulation of root growth.