With the development of large language models, multiple AIs have become available for code generation (such as ChatGPT and StarCoder) and are adopted widely. It is often desirable to know whether a piece of code is generated by AI, and furthermore, which AI is the author. For instance, if a certain version of AI is known to generate vulnerable codes, it is particularly important to know the creator. Watermarking is broadly considered a promising solution and is successfully applied for identifying AI-generated text. However, existing efforts on watermarking AI-generated codes are far from ideal, and pose more challenges than watermarking general text due to limited flexibility and encoding space. In this work, we propose ACW (AI Code Watermarking), a novel method for watermarking AI-generated codes. The key idea of ACW is to selectively apply a set of carefully-designed semantic-preserving, idempotent code transformations, whose presence (or absence) allows us to determine the existence of watermarks. It is efficient as it requires no training or fine-tuning and works in a black-box manner. Our experimental results show that ACW is effective (i.e., achieving high accuracy on detecting AI-generated codes and extracting watermarks) as well as resilient, significantly outperforming existing approaches.