The results of the an experimental study of the rolling contact fatigue of gas and vacuum carburized 4320, 5120, 8620 and 9310 steels will be presented. The steels were carburized to yield an equivalent carbon concentration profile, hardness profile and case microstructure. The results are analyzed with a Weibull analysis. The results are discussed in terms of microstructure and crack initiation mechanism. The roll of residual stress distribution and inclusions are also presented.