Blogspace is a primary example of online social networks. In blogspace, there are a number of communities, each of which consists of members having dense relationships with one another. In this paper, we address formation and evolution of blog communities. We first make two claims: (1) a high level of contents similarity of blogs increases the likelihood of their belonging to the same community in a blog network; (2) A high level of contents similarity of communities increases the possibility of their being merged into a bigger community in the future. We verify these claims by analyzing a large volume of real-world blog data of 2 millions bloggers through extensive experiments. Our results provide new insights towards subsequent research on formation and evolution of communities in a blog network.