This paper presents the degradation data analysis on a lithium ion battery. Electric charge and discharge test are performed for 8 samples, and their capacitances are measured repeatedly at prespecified times. From the capacitance-time graph, it is shown that the capacitance is decreasing over time. For each specimen, the degradation model minimizing mean square error is selected among linear, exponential, logarithm, and power models, and the failure time at which the capacitance leads to a critical value is predicted from the model. Through goodness-of-fit tests, we have assumed that the failure times follow lognormal distribution, and estimated reliability measures of the lithium ion battery.