Microblog (e.g. Twitter) is fast emerging social medium for information diffusion. This paper presents an analysis of the pattern of information diffusion in microblog. We have collected the data of 1,798,901 tweets from Sina microblog service- the Chinese equivalent of Twitter. Our analysis unveils that the distribution of tweets' popularity follow the stretched exponential (SE) model. Based on this observation we analyse a multiplicative cascade model for describing tweets popularity.