This study analyzed authorsuse of theory in 654 articles that appeared in two core library and information science journals during last three decades. In order to analyze degree of theory use of LIS, such as, publication productivity, growth and distribution of theory in subfields, name and origin of theory, usability of each theory, subfields and journals, and so on, content analysis of LIS theories was performed through conceptual and empirical study. For the purpose of this study, we suggested a couple of new analytical methods, so called, 'Subfield Classification Scheme' within LIS, and '5 Degrees of Theory Use' model for the first time.