Abstract: Based on experimental data of positron annihilation technology, electrolytic dissolution technique, electron back-scattered pattern, etc. And by analysis the strengthening factors, the strengthening mechanism of ultra-thin hot strip of low carbon steel produced by CSP (Compact Strip Production) technique was investigated. The value of each strengthening mechanism and its contribution percentage to yield strength were achieved. The results show that refinement strengthening is the predominant strengthening mode; precipitation strengthening and dislocation strengthening are second to it, their contributions to yield strength are almost equal.