Brassinosteroids (BRs) control virtually every aspect of plant growth and development. BRs act alone or with other exogenous and endogenous signals including auxin and light. To screen for the novel player involved in BR signaling in Arabidopsis, we employed cDNA overexpression strategy. We created a cDNA library to be expressed under the 35S overexpression promoter, and introduced into a weak brassinosteroid insensitive 1 (bri1) mutant. The mutant dubbed bri1-5 with long petiole (blp) was identified to display bigger stature especially in hypocotyl and petiole length relative to bri1-5. Sequence analysis of the rescued transgene revealed that blp consisted of a chimeric DNA consisting of a 3’ half of PHYB, 2 bp insertion, and a part of a chloroplast ribosomal RNA. Re-introduction of chimeric DNA into bri1-5 recapitulated blp phenotype. The blp phenotypes being similar to phyB mutants led us to examine both the PHYB transcript and protein levels in the blp 35Spro:PHYB doubly homozygous line. Lower levels of both transcripts and proteins of PHYB suggested that introduction of the chimeric gene interfered with the stability of PHYB transcripts. Our results highlight that overexpression mutagenesis facilitates functional genomics to decipher a function of Arabidopsis genome.