Abstract: A model for hydrogen induced embrittling process sone(EPZ)of aching is proposed and developed,based on the notion that hydrogen enters the EPZ ahead of the crack-tip and promotes localized plastic now.Formulae for the threshold stass intensity factor in hydrogen assistal crack growth,KIH,and the relationships between hydrogen environment pressure and KIH are first derived.Both qualitative and quantitative analysis are in agreement with experimental data on superhigh strength steel AISI 4340.