Brassinosteroids are plant steroid hormones work for plant growth and development. We had identified brassinosteroid biosynthesis inhibitor Brz insensitive mutant bill (Brz-insensitive-long hypoeotyl 1) from EMS-mutation lines, and bil5 from fast-neutron lines. We tried to screen new bil mutants from Arabidopsis activation-tag lines that showed longer hypocotyls than wild type even with Brz in the dark, and selected semidominant mutant bil3. bil3 showed slender dwarf phenotype related to excessive epinastic leaves and thin inflorescences in the light. We are now establishing transgenic plants that overexpress the candidate genes to identify relation between the genes and bil3 mutants. In the comparison these mutants with mutation genes, we would like to clarify in detail the mechanism of brassinosteroid signaling transduction.