Auxin is a group of small natural and synthetic molecules having diverse regulatroy funtions in plant growth and development. In this review, two auxin binding proteins identified by biochemical experiements to measure their auxin binding activities and biochemical funtions are described. ABP1, a 22 kDa auxin binding protein, shows strong auxin binding affinity and possibly plays an important role in plant development, althouth its biochemical funtion are still unclear. ABP57, a 57 kDa soluble protein from rice shoots, has both of IAA binding activity and the plasma membrane proton pump activation. Although it is yet to be accomplished, the improvement of agronomic traits using auxin binding proteins is worth to be considered, since auxin is known to be related to such a diverse crop traits.