The　objective　of　this　work　is　to　study　the　effect　of　structural　size　on　the　seismic　behavior　of　the　square　concrete　-filled　steel　tubular　(CFST)　short　columns　experimentally.　A　total　of　eight　CFST　short　columns　with　different　structural　sizes　(maximum　column　width　of　800　mm)　were　designed　and　tested.　All　columns　have　a　constant　width-to-thickness　ratio　B/t　of　50,　an　axial　load　ratio　n　of　0.4,　and　a　slender　ratio　L/B　of　2.0.　The　seismic　per-formances　of　the　columns　were　tested　and　analyzed,　involving　the　failure　pattern,　the　hysteretic　characteristic,　the　ductility　capacity,　the　stiffness　degradation　and　the　energy　dissipation.　The　typical　compressive-flexural　failure　pattern　was　observed　for　all　tested　columns,　and　the　local　buckling　of　the　steel　tube　and　the　crushing　of　the　core　concretes　were　also　found　at　the　bottom　of　the　column.　With　the　increase　of　the　cross-sectional　width,　the　energy　dissipation　capacity　and　the　ductility　capacity　of　the　square　CFST　short　columns　decrease.　In　addition,　it　is　found　that　as　the　cross-sectional　width　of　the　square　CFST　short　column　increases,　the　nominal　flexural　strength　decreases　significantly,　exhibiting　a　remarkable　size　effect.　As　the　cross-sectional　width　varies　from　200　mm　to　800　mm,　the　peak　flexural　stress,　i.e.　the　nominal　flexural　strength,　decreases　by　46.9%.　Finally,　the　comparisons　of　the　experimental　results　were　made　against　some　design　codes　including　GB50936,　AISC-360　and　EC4.　It　is　found　that　most　design　codes　without　consideration　of　size　effect　would　be　unsafe　for　the　larger-sized　columns.