Title: Effective Cross-instance Positive Relations for Generalized Category Discovery. Abstract: We tackle the issue of generalized category discovery (GCD). GCD considers the open-world problem of automatically clustering a partially labelled dataset, in which the unlabelled data contain instances from novel categories and also the labelled classes. In this paper, we address the GCD problem without a known category number in the unlabelled data. We propose a framework, named CiP, to bootstrap the representation by exploiting Cross-instance Positive relations for contrastive learning in the partially labelled data which are neglected in existing methods. First, to obtain reliable cross-instance relations to facilitate the representation learning, we introduce a semi-supervised hierarchical clustering algorithm, named selective neighbor clustering (SNC), which can produce a clustering hierarchy directly from the connected components in the graph constructed by selective neighbors. We also extend SNC to be capable of label assignment for the unlabelled instances with the given class number. Moreover, we present a method to estimate the unknown class number using SNC with a joint reference score considering clustering indexes of both labelled and unlabelled data. Finally, we thoroughly evaluate our CiP framework on public generic image recognition datasets (CIFAR-10, CIFAR-100, and ImageNet-100) and challenging fine-grained datasets (CUB, Stanford Cars, and Herbarium19), all establishing the new state-of-the-art.