Abstract: The relationship between tempering temperature andcrack initiation life for 40Cr steel is investigated by system analysismethod in this paper. The samples at three tempering temperatures are examined in three-point bending fatigue tests. The results showthat there are two stages of crack initiations varing with tempertemperatures: (1) from 20^\circC\sim450^\circC, the crackinitiation life increases remarkably; (2) from 450^\circC\sim580^\circC, the crackinitiation life increases insignificantly. The results also show that thesystem analysis method is effective in the measurement of crack initiation lifenumerically.