Brassinosteroids (BRs) are a group of steroidal phytohormones, playing critical roles in almost all physiological aspects during the life span of a plant. In Arabidopsis, BRs are perceived at the cell surface, triggering a reversible phosphorylation-based signaling cascade that leads to the activation and nuclear accumulation of a family of transcription factors, represented by BES1 and BZR1. Protein farnesylation is a type of post-translational modification, functioning in many important cellular processes. Previous studies demonstrated a role of farnesylation in BR biosynthesis via regulating the endoplasmic reticulum localization of a key bassino-lide (BL) biosynthetic enzyme BR6ox2. Whether such a process is also involved in BR signaling is not un-derstood. Here, we demonstrate that protein farne-sylation is involved in mediating BR signaling in Arabidopsis. A loss-of-function mutant of EN-HANCED RESPONSE TO ABA 1 (ERA1), encoding a β subunit of the protein farnesyl transferase hol-oenzyme, can alter the BL sensitivity of bak1-4 from a reduced to a hypersensitive level. era1 can partially rescue the BR defective phenotype of a hetero-zygous mutant of bin2-1, a gain-of-function mutant of BIN2 which encodes a negative regulator in the BR signaling. Our genetic and biochemical analyses re-vealed that ERA1 plays a significant role in regulating the protein stability of BES1.