Title: Automatic Curriculum Generation for Reinforcement Learning in Zero-Sum Games. Abstract: Curriculum learning (CL), whose core idea is to train from easy to hard, is a popular technique to accelerate reinforcement learning (RL) training. It has also been a trend to automate the curriculum generation process. Automatic CL works primarily focus on goal-conditioned RL problems, where an explicit indicator of training progress, e.g., reward or success rate, can be used to prioritize the training tasks. However, such a requirement is no longer valid in zero-sum games: there are no goals for the agents, and the accumulative reward of the learning policy can constantly fluctuate throughout training. In this work, we present the first theoretical framework of automatic curriculum learning in the setting of zero-sum games and derive a surprisingly simple indicator of training progress, i.e., the Q-value variance, which can be directly approximated by computing the variance of value network ensembles. With such a progression metric, we further adopt a particle-based task sampler to generate initial environment configurations for training, which is particularly lightweight, computation-efficient, and naturally multi-modal. Combining these techniques with multi-agent PPO training, we obtain our final algorithm, Zero-sum Automatic Curriculum Learning (ZACL). We first evaluate ZACL in a 2D particle-world environment, where ZACL produces much stronger policies than popular RL methods for zero-sum games using the same amount of samples. Then we show in the challenging hide-and-seek environment that ZACL can lead to all four emergent phases using a single desktop computer, which is reported for the first time in the literature. The project website is at https://sites.google.com/view/zacl.