Up till the end of 2009，the number of Chinese netizens had reached 384 million. 61.5%（about 236 million people）of them are below age 29，and 32.9%（about 126 million people）are below age 19. The ratio of younger-age（under age 10）netizens increased notably，up from 0.4% in 2008 to 1.1%. Meanwhile，online communities have undergone rapid development in recent years. The online communities established in 2007-2009 account for 73.5% of the total. According to a statistical report in 2006，teenager users under age 25 make up 46.9% of all the community users. These data show that teenagers constitute the largest user group of Chinese internet and online communities alike. Owing to their advancement of personal interactions，internet communities are producing even greater effects on teenagers. It is thus worthy of our note to explore the formation and development of teenagers’ online communities，learn the laws of their development，and examine the characteristics of teenagers’ involvement in such interactions，with a view to guiding the growth of teenagers in the information age. Drawing on sociological theories on community，this study aims to describe the formation and development of teenagers’ online communities and examine the possible tendencies，so as to contribute to the building of a healthy and harmonious online community for teenagers.