Title: Impact of the Last Fully Connected Layer on Out-of-distribution Detection. Abstract: Out-of-distribution (OOD) detection, a task that aims to detect OOD data during deployment, has received lots of research attention recently, due to its importance for the safe deployment of deep models. In this task, a major problem is how to handle the overconfidence problem in OOD data. While this problem has been explored from several perspectives in previous works, such as the measure of OOD uncertainty and the activation function, the connection between the last fully connected (FC) layer and this overconfidence problem is still less explored. In this paper, we find that the weight of the last FC layer of the model trained on in-distribution (ID) data can be an important source of the overconfidence problem, and we propose a simple yet effective OOD detection method to assign the weight of the last FC layer with small values instead of using the original weight trained on ID data. We analyze in Sec.5 that our proposed method can make the OOD data and the ID data to be more separable, and thus alleviate the overconfidence problem. Moreover, our proposed method can be flexibly applied on various off-the-shelf OOD detection methods. We show the effectiveness of our proposed method through extensive experiments on the ImageNet dataset, the CIFAR-10 dataset, and the CIFAR-100 dataset.