A method for estimating a service life of a motor that is driven by variable duty cycle control. The method includes calculating an elapsed service life ratio representing a ratio of an elapsed service life to all service life according to the following formula to estimate the service life of the motor: L s = ∑ i ⁢ ⁢ ( t i L i × ( 1.5 ) m ) ⁢ ⁢ m = 40 - max ⁡ ( 40 , K i ) 10 , where Ls represents the elapsed service life ratio, ti represents a driving time in a section i, Li represents a service life expectancy at a duty cycle Di in the section i and at an environmental temperature of the motor being 40° C., and Ki represents a temperature in the section i.