Online social network is becoming more and more popular these days. Some literatures about it have been published, but few papers talk about Sin a Weibo, which is the most prevalent micro log in China and has more than 100 million users. The goal of this paper is to study the topological characteristics of Sin a Weibo and show what people use Sin a Weibo for. There are two main researches in micro logging system: (i) How is the overlay formed, and (ii) How does the information transmit. We crawled the Sin a micro log for one month and got 1.12 million user profiles. We mainly study the topology of Sin a micro log, and find that: (1) the overlay of Weibo is dynamic, (2) most of the links between users are one-way, (3) there is a core network in Sin a Weibo, and (4) the radius of Sine Weibo is very short, which make it different from human social networks and other online social networks. Finally, we classify online social networks into two types according to our results. To the best of our knowledge this work is the first quantitative study on the topological characteristics of Sin a Weibo, and the results can make people know Sin a Weibo clearly.