Studies measuring online learning have adopted different perspectives, resulting in different approaches to their assessment of online learning. However, when we consider the literature from a wider angle, there may be complimentary or contrasting relationships. This study performed content analysis on a total of 44 studies that used questionnaires with good reliability and validity from the database of the Social Science Citation Index. Through encoding the 241 factors and 1,171 items used in the questionnaires, we found relationships among them that could be supported by classical theoretical frameworks or our general knowledge of instruction. The content measured by these questionnaires can be divided into five categories according to Li's (2001) theory of instructional elements: (1) learning environment, (2) learner, (3) teacher, (4) learning content, and (5) learning interaction. Following the initial analysis, this study used the unified theory of acceptance and use of technology (UTAUT) (Oye et al., 2014) to classify 58 factors that reflect the learning environment. In addition, the combination of Marzano and Kendall's (2007) new taxonomy of educational objectives and learning outcomes provides useful information for 111 factors used to assess learners.