In metal cutting processes, the use of cutting fluid is the most common strategy to improve the tool life, the product surface finish and the size accuracy. But cutting fluid may result in a series of negative effects on human health and environment. In order to apply cutting fluid reasonably, little quantity lubricant cooling mechanisms under condition of liquid forced convection were researched based on Prandtl convective heat transfer boundary layer theory, the formula of minimum cutting fluid flow rate desired by sufficient cooling under condition of liquid forced convection was derived, and the validity of Prandtl convective heat transfer boundary layer theory on aluminum alloy 7050-T7451 milling process was verified through experiments. Experimental result shows that little quantity lubricant cooling mechanism may be analyzed reasonably by Prandtl convective heat transfer boundary layer theory.