Estimating reliability of a non-repairable system using the degradation data, constant or time-variant variance assumption could affect results of reliability estimation. This paper showed reliability estimation and comparison results under normal conditions using accelerated degradation data , according to variance assumption of the data at each measurement time. Degradation data from three accelerated conditions with stress factors of temperature and humidity were used to estimate reliability. The B10 lifetime was estimated as 1243.8 years for constant variance assumption, and 18.9 years for time-variant variance. In additionally, variance assumption provided different important analysis results of stresses to reliability. Thus, accurate assumption of variance at each measurement time is required when estimating reliability using degradation data of a non-repairable system.