A lubricating oil additive containing at least one acrylic block copolymer having a weight-average molecular weight of 10,000 to 1,000,000 and a molecular weight distribution [ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), i.e., Mw/Mn ratio] of 1.5 or below; and lubricating oil compositions containing the additive. The lubricating oil additive exhibits a thickening effect higher than those of conventional acrylic lubricating oil additives and can improve the viscosity index of lubricating oil even when added in a small amount. Further, the additive can also improve the low-temperature characteristics, flow characteristics, and shear stability. The lubricating oil compositions enable energy saving and can lengthen the lifetime of lubricating oil and reduce the load on the global environment.