A method for producing grains of aliphatic acid silver salt by mixing a solution containing silver ions and a solution of aliphatic acid alkali metal salt to attain reaction of them, wherein (1) 95-100% of the total molar number to be added of the aliphatic acid alkali metal salt is added under a condition that dilution factor of the solution of aliphatic acid alkali metal salt becomes 10-fold or higher at 1 second after the addition, (2) 95-100% of the total amount to be added of silver in the solution containing silver ions is added under a condition that dilution factor of the solution containing silver ions becomes 20-fold or higher at 1 second after the addition, and (3) 50-100% of the total amount to be added of the solution of aliphatic acid alkali metal salt is added simultaneously with the solution containing silver ions. According to the present invention, there is provided a method for producing grains of aliphatic acid silver salt, which show excellent fog prevention ability, in addition, excellent fog prevention ability even after time lapse, when used in a thermally processed image recording material.