An injection-molding polyolefin composition suitable for obtaining an injection-molded article having excellent sliding property and excellent impact resistance and being free from delamination, said polyolefin composition being characterized in that(1) the composition consists substantially of an ultrahigh-molecular-weight polyolefin having an intrinsic viscosity of 10-40 dl/g as measured in decalin of 135.degree. C. and a low-molecular-weight to high-molecular-weight polyolefin having an intrinsic viscosity of 0.1-5 dl/g as measured in decalin of 135.degree. C.,(2) the ultrahigh-molecular-weight polyolefin is contained in the composition in an amount of 10-40% by weight based on the total weight of the ultrahigh-molecular-weight polyolefin and the low-molecular-weight to high-molecular-weight polyolefin,(3) the composition has an intrinsic viscosity [.eta.]c of 3.5-15 dl/g as measured in decalin of 135.degree. C.,(4) the composition has a melt torque T of 4.5 kg.cm or below, and(5) the section of the injection-molded square plate produced from the composition has a phase-separated structure composed of a continuous phase and a discontinuous phase and the average paticle diameter (average major axis length) of the discontinuous phase is 20 .mu.m or below.