To prepare a mixture of fatty acids with a comb polymer that can improve the lubricity of a middle distillate fuel oil as an additive and that is also excellent in low-temperature stability.

The fatty acid mixture having low-temperature stability comprises (A) 10-99.99 wt.% mixture of fatty acids comprising (A1) 1-99 wt.% one or more saturated mono- or di-carboxylic acids and (A2) 1-99 wt.% one or more unsaturated mono- or di-carboxylic acids and (B) 0.01-90 wt.% copolymer comprising (B1) 40-60 mol% divalent structural unit of formula (B1 a) or (B1b) (wherein X is 0 or N-R1; (a) and (b) are each 0 or 1; and a+b=1) and (B2) 60-40 mol% divalent structural unit of the formula: H2C-CR2R3. The mixture is used for improving the lubricity of a low-sulfur middle distillate.