Abstract: A lubricating oil composition comprising:(1) 98.6 to 53% by weight of a base oil having a kinematic viscosity of 3 to 20 centistokes as determined at 100.degree. C.;(2) 0.2 to 5% by weight of a sulfurized oxymetal organophosphorodithioate represented by the formula (I): ##STR1## and/or sulfurized oxymetal dithiocarbamate represented by the formula (II): ##STR2## (3) 0.1 to 7% by weight of at least one zinc dithiophosphate represented by the formula (III): ##STR3## (4) 0.1 to 20% by weight of a calcium alkylbenzenesulfonate and/or a calcium petroleumsulfonate; and(5) 1 to 15% by weight of alkenylsuccinic acid imide and/or boron derivatives thereof. The present lubricating oil composition is superior in reducing the mechanical friction loss of four-cycle engines, particularly small-sized high power four-cycle engines when used as an engine oil.