In the intelligent age, human factors, as a discipline, can effectively help understand and predict the process of human-AI interaction and solve various problems that arise in human-AI interaction. This chapter proposes that human factors in the AI age should handle four fundamental relationships in human-AI interaction: (1) AI functioning as an unseen service provider behind the scenes; (2) humans interacting with AI-based intelligent agents with low involvement when the agent’s identity is explicit; (3) humans teaming up with intelligent agents; and (4) humans being managed by intelligent agents. Each of these relationships involves, to varying degrees, four core scientific issues: optimizing human-computer interaction through intelligent agents, human perception of intelligent agents as e physical entities, human comprehension of intelligent agents, and the emotional interaction between humans and intelligent agents. It is argued that human factors can play a pivotal role in advancing human-centered AI in the intelligent era by seamlessly integrating classic human factors methods, emerging technological approaches, and innovative design thinking.