By　observation　and　analysis　of　chemical　composition,　microstructure,　nitrogen　oxygen　content　detection,　SEM　(scanning　electron　microscope)　and　EDS　(energy　dispersive　X-ray　spectrometry),　the　cause　of　longitudinal　crack　on　400E　hot　rolled　ribbed　bar　surface　was　explored.　Combining　with　production　process,　the　influence　mechanism　of　chemical　composition,　non-metallic　inclusion,　widmanstatten　structure　and　nitrogen　oxygen　content　on　the　cause　of　longitudinal　crack　on　the　bar　surface　was　discussed.　The　results　show　that　the　main　reason　for　the　longitudinal　crack　is　due　to　excessive　C　content　of　the　steel,　non-metallic　inclusions,　widmanstatten　structure,　and　higher　contents　of　oxygen　and　nitrogen.　©　2016,　Editorial　Office　of　＂Jinshu　Rechuli＂.　All　right　reserved.