Abstract: Spontaneous imbibition is of special importance to the efficiency of oil recovery from fractured reservoirs.Many factors affect the process of spontaneous imbibition.This paper is focused on the effect of initial water saturation and rock lithology on rate of imbibition and oil recovery.The porous media used in this study are high permeability sandstone,low permeability sandstone and chalk.Results show that initial water saturation s_wi has a strong effect on rate of imbibition of sandstone system.For s_wi<6%,imbibition rate decreases with increase in s_wi.In this range of s_wi,6%~l5%,the rate of imbibition is at a minimum.For 30%>s_wi>15%,imbibition rate increases with increase in s_wi.Among three types of rocks,at zero initial water saturation,imbibition recovery from chalk samples is the highest,about 23% higher than that from sandstone with similar permeability.