The linear Wiener process–based degradation model is commonly used for the lifetime assessment and remaining useful life estimation. This article addresses the effects of model mis-specification of the linear Wiener process for the remaining useful life estimation. First, we study the model mis-specification effects on the parameters’ estimation and the lifetime distribution. Then, the effects of model mis-specification on the remaining useful life estimation and the predictive maintenance decision-making are analysed through some numerical examples and a case study. The results show that mis-specifying the linear Wiener process without measurement error as that with measurement error is negligible. However, under the inverse condition, the mis-specification could result in premature maintenances or failure maintenances, which increases the maintenance costs.