This paper explores trust of mobile applications based on users' behaviors. It proposes a trust behavior construct through principal component analysis, reliability analysis, and confirmatory factor analysis based on the data collected from a questionnaire survey with more than 1,500 participants. It is indicated that a user's trust behavior is composed of three principal constructs: using behavior, reflection behavior, and correlation behavior. They are further delineated into 12 measurable sub-constructs and relate to a number of external factors. The data analysis showed that the questionnaire has positive psychometric properties with respect to construct validity and reliability. We also discuss the practical significance and limitations of our work toward usable trust management.