MOOCs, which stands for Massive Open Online Courses, have attracted millions of users around the world and it has a promise to be a very important part of future education. However, there is little research on users' adoption of MOOCs. This paper aims to improve the understanding of users' behavior intention to use MOOCs. The proposed research model is an extension of technology acceptance model with three factors from the institutional theory. And an empirical study with 247 subjects was conducted to test this model. The results indicate that both perceived usefulness and perceived ease of use directly affect users' behavior intention to use MOOCs significantly. Another interesting finding is that mimetic pressures also have a significant positive influence on users' behavior intention to use MOOCs.