Abstract: With the breakthrough of mobile communications and Web technology, online social network led by microblog has developed widely in China. More and more people begin to share information and opinions through microblogging system. In order to gain insights into the topological characteristics of microblogging network and online user behavior characteristics, we launch an active measurement of Sina microblog which is the biggest microblogging system in China. This paper analyzes the results from our measurement and investigates on topological characteristics and user behavior patterns in Sina microblog, compared with existing results via measuring other online social networks. Our major findings include: 1) Sina microblog has apparent smalll-world effect; 2) While the indegree of Sina microblog follows power-law distribution, the outdegree distribution appears to have multiple separate power-law regimes with different exponents; 3) Unlike other online social networks, Sina microblog has weak correlation of indegree and outdegree; 4) The overlay graph of Sina microblog appears assortative mixing; 5)Tweeting time of users exhibits daily and weekly patterns; 6) The number of tweets in Sina microblog approaches Weibull distribution; 7) Actions of retweeting and replying have strong correlation in Sina microblog, and the probability of retweeting is higher than that of replying. These research and findings will be helpful not only for designing mathematical or computational models which are coincident with actual microblogging characteristics in China, but also for monitoring,directing and dominating of microblogging system.