This paper introduces a new opinion model using space-time elements. It proposes the concept of an Opinion Importance Factor which is composed of a Time Importance Factor and a Source Importance Factor. Based on that, it analyzes the characteristics of the Time Importance Factor in different applications and divides the Source Importance Factor into two parts, the influence of source and the relatedness between source and domain. The experimental results show that space-time elements are important, and taking account of them can expand the scope of application of opinion mining and achieve more convincing results.