Blog has been one of the fastest growing applications in the Internet. Content provided by users has also been an important source of information. Current studies, however, can not explain well why people build their blog. Based on the Task-Technology Fit (TTF) model, this study examines both the impacts of task and technology characteristics on user evaluation of blog and blog usage. Task needs for using blog are described as selfpresentation, self-orientation, and sociality need; technology characteristics refer to the perception of forward and backward social presence. Results of this study show that selfpresentation, sociality need, and the perception of social presence can explain why people build their blog. Interactions between task and technology features also affect user evaluation and blog usage, as predicted by the TTF model.