The article presents the need for formalization of sociological theories. Formalization is needed to test sociological theories, that is to define if theories fit data of researches, and, respectively, if theories describe or explain social reality well. Theory is considered as a set of concepts (that may be empirically measured) and relations between the concepts that in total describe or explain certain part of social reality; theory is represented as causal model which consists of a set of interrelated hypotheses. Formalization is regarded as the translation of verbal statement of the theory to the language of mathematical formulae. Difficulties and limitations of formalization of sociological theories are considered. Some technical aspects of formalization are specified due to use of quantitative data in verification process. General guidelines for formalization of sociological theories and rules of formalization of separate theoretical statements are inferred on the basis of works of researhers of theory formalization (H.Blalock, J.Gibbs, H.Zetterberg). Three cases of formalization of sociological theories and stages of formalization within each of these cases of formalization are suggested. These three cases are: a) formalization of sociological theories from verbal theories of sociologist(s) by researcher (group of researchers); b) formalization of theoretical assumptions of researcher when conducting sociological research; c) (re)formalization of theories that was made by other researchers in the past aimed to adapt to specific data.