In a method for predicting deterioration of grease, the grease is applied between a semiconductor module and a cooler. The semiconductor module accommodates a semiconductor element. The method for predicting deterioration includes predicting deterioration of the grease after specified heat cycles by using: a variable G1/G2 that is acquired by dividing an initial storage modulus G1 of the grease by an initial loss modulus G2 of the grease at an expected maximum use temperature of the semiconductor element; and distortion dD of the grease at the time when the initial storage modulus G1 and the initial loss modulus G2 have the same value.