Abstract: In order to acquire the influence of boron on mechanical properties of carbon structural steels, through counting and analyzing chemical composition and mechanical properties of boron micro-alloyed hot-rolled H-beam and ordinary carbon structural steels H-beam, the influence of boron on properties and structure of hot-rolled H-beam was studied, and the influence of different composition of boron on the toughness of production was sum up. The resulted showed that the trace boron prevented the aliquation of element P and S, restrained embrittlement of the grain boundary, thus greatly improved the toughness of hot-rolled H-beam production.