Abstract: Implant test in Z-direction was used to investigate the sensitivity of 15MnVN and 15MnVNCu steels to cracking in that direction.Experimental results showed that the critical stress of implant in Z-direction(σ_cr)_z a was dependent on P_CM of the base metal and the hydrogen content.This is just the same as in the case of implant test in X-direction,which reflects the essence of hydrogen induced delayed cracking.By means of fractographic analysis and acoustic emission technique,the authors confirmed that(σ_cr)_z a indicated mainly the sensitivity to hydrogen embrittlement of steels rather than the sensitivity to lamellar tearing on our experimental conditions and the conditions would have an effect upon the significance of implant test in Z-direction.The role played by platelike M nS inclusions in the initiation and propagation of hydrogen induced delayed cracking was discussed and a comparison of the advantages and disadvantages of using rupture-criterion or crack-initiation-criterion was made.