This invention provides an oil composition for cutting and grinding by minimum quantity lubrication system, characterized by comprising an ester oil with a kinematic viscosity of 0.5-20 mm2/s at 100° C., and an ester-based polymer with a kinematic viscosity exceeding 20 mm2/s at 100° C. and an average molecular weight of 5,000-10,000,000. The oil composition for cutting and grinding by minimum quantity lubrication system according to the invention can achieve an excellent balance between misting property and inhibition of floating mist and ensure that an adequate amount reaches the working section, for cutting and grinding by minimum quantity lubrication system.