The LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) genes encode proteins harboring a conserved amino acid domain, referred to as the LOB domain. Recent studies have revealed developmental functions of some LBD genes in Arabidopsis and crop plants, but the biological functions of many other LBD genes remain to be determined. Our recent study has shown that LBD18 functions in lateral root formation in combination with LBD16 downstream of AUXIN RESPONSE FACTOR7 (ARF7) and ARF19 in auxin signaling. In this study, we showed that LBD18-transgenic lines expressing high levels of the transcripts in steroid regulator inducible system exhibit growth inhibition of aerial parts of the plants and epinastic symptoms on the newly formed leaves upon treatment of dexamethasone (DEX). We found that AINTEGUMENTA (ANT) known as one of the main controllers of plant organ size and PLETHORA (PLT) homologues essential for root development were downregulated by DEX treatment. These results implicate that LBD18 might be involved in the regulation of plant organ size and root development via the ANT and PLT genes. In addition, we showed that ANT can be downregulated by DEX in the presence of cycloheximide, a protein synthesis inhibitor, indicating that ANT might be a direct target gene for LBD18.