This paper intends to investigate how the differentiation between traditional and alternative media is reinforced through the mutual attention among journalists, based on field theory. It further argues that a significant social movement can serve as a critical event which intensifies mutual monitoring among journalists. Focusing on the anti-extradition law amendment bill movement in Hong Kong, this study conducted interviews with 20 reporters from traditional and alternative media. The analysis revealed two mechanisms of mutual monitoring: (a) direct monitoring at protest sites and (b) monitoring published works. Such monitoring activities reinforce the differentiation between traditional and alternative media through encouraging boundary work exercised by traditional media journalists and providing cues for alternative media reporters to cover important perspectives which are missing in the mainstream coverage.