Abstract: The carbide composition, quantity and particle size distribution of HSLC steel produced by CSP process were studied by chemical phase analysis and SAXS method with a 3014 X-ray diffracto-spectrometer and a Kratky small angle scattering goniometer. It is found that the content of carbide with its size less than 18 nm is more than that of Nb(C_xN_y) with its size less than 18 nm in Nb-contained CSP-HSLA steel reported in reference. The strengthening effect of nano-scale carbide on the steel was discussed.