The aim of this thesis is to explore the ways in which class aspects of trade unions are formed in relation to their organisational properties. To achieve this aim, various assertions on the fundamental role of trade unions are first discussed, and criticised for treating trade unions as organisations that typically react to external stimuli and not as organisations that also generate goals and strategies on their own initiative. Then the focus moves to class consciousness as an effective measure of class capacity of both trade unions and their members, and in doing so, the concept of class consciousness is reformulated to embrace collective and action-oriented aspects of it. Having tested this reformulated concept initially by analysing the comparative characters and capacities of three national unions at different points in time in Korea, a class consciousness scale is developed to measure not only diverse aspects of class consciousness but also qualitatively different levels of it, and such research methods as text analysis and interviews are also employed for the main empirical survey of this thesis. The chief findings include the uneven development of class consciousness within as well as between the individuals, and the conspicuousness of the leadership effect/alliance factor, the less-then impressive effects of gender, size, union age, strike experience, and nationality of ownership on the overall level of class consciousness. Simultaneously, the potentially intriguing implications of the specific influences of those factors on certain aspects of class consciousness are discussed. The thesis is concluded with a recapitulation of the significance of the organisational factors investigated, suggestions on possible directions for further studies, and an urge to study trade unions as class organisations, with the emphasis on both words.