The objective of the present study is to clarify the effect of slip conditions, such as slip ratio, outer surface velocity, and lubricant oil temperature on the contact fatigue strength for pitting (pitting strength) of carburized steels. Rolling contact fatigue (RCF) tests were conducted under oil lubrication on 26mm-diameter specimens made of a carburized steel with various combinations of slip ratio, outer surface velocity, and lubricant oil temperatures. Relationships among those three parameters were also evaluated. The pitting strength decreased with the increase of slip ratio, with the decrease of outer surface velocity, and with the decrease of lubrication oil temperature. It was found that the slip ratio, the outer surface velocity, and the lubricant oil temperature influenced the friction coefficient and the contact surface temperature. The friction coefficient increased when slip ratio increased. The friction coefficient was considered to be the dominant factor which directly affected the pitting strength.