Title: SPIDER: Searching Personalized Neural Architecture for Federated Learning. Abstract: Federated learning (FL) is an efficient learning framework that assists distributed machine learning when data cannot be shared with a centralized server. Recent advancements in FL use predefined architecture-based learning for all the clients. However, given that clients' data are invisible to the server and data distributions are non-identical across clients, a predefined architecture discovered in a centralized setting may not be an optimal solution for all the clientsin FL. Motivated by this challenge, we introduce SPIDER, an algorithmic framework that aims to Search Personalized neural architecture for feDERated learning. SPIDER is designed based on two unique features: (1) alternately optimizing one architecture-homogeneous global model (Supernet) in a generic FL manner and one architecture-heterogeneous local model that is connected to the global model by weight-sharing-based regularization (2) achieving architecture-heterogeneous local model by an operation-level perturbation based neural architecture search method. Experimental results demonstrate that SPIDER outperforms other state-of-the-art personalization methods with much fewer times of hyperparameter tuning.