Logical structure of N. Luhmann sociology is shown as an enrichment of the discipline by general scientific achievements of systems theory and building on this basis an “observing” theory of society. Results of Luhmann’s creativity are presented on preliminary and main phases beginning with the book “Social systems” and culminating in the work “Society of societies”. The conclusion is grounded that it represents the second stage in the development of system-theoretical paradigm in sociology after Parsons.