The purpose of the article. The article covers the mechanisms of behavioural economics introducing into the state policy of the country. This problem becomes especially relevant in modern conditions as people tend to make unreasonable economic decisions. The purpose of this paper is to research the irrational motives of individuals’ behaviour in making economic decisions and to determine the measures for introducing the mechanisms of behavioural economics in the state policy of Ukraine. Methodology. The survey is based on the analysis of the scientific papers of the following scientists in economics: A. Smith, K. Marks, J. Bentham, M. Weber, J. Schumpeter, A. Marshall, J. Keynes, H. Simon, H. Becker, V. Smith, D. Kahneman, A. Tversky, Richard H. Thaler, G. Akerlof, R. Shiller; on the empirical analysis and on the analysis of the behaviour economic instruments. The objective of the article is the survey of irrational motives of human behaviour during the decision-making process in economic conditions, to determine the influence of the irrational motives on social and economic development and to develop measures and activities for implementation of behavioural economics mechanisms in the state policy. The subject of the article is theoretical and methodological aspects of the behavioural economics mechanisms in the state policy. The results. The research in the article shows and explains the main trends of the behavioural studies in the modern condition, in particular: behavioural finance, behavioural game theory, the problem of making a choice in time, prospect theory. One of the examples of Herding behaviour shown in the article is the peculiarities of behavioural aspects of financial markets. Special attention is given to the “nudge theory” used by the leading countries. Behavioural economics possess the instruments, which maintain the decent living of the population and sustainable social and economic situation in the society. The results of research represent that the behavioural peculiarities of the society together with the interaction with the state policy result in the distrust of the society to the authority’s representatives. Besides, the low-income level of the population leads to the predominance of survival values in the society, while in the highly developed countries, the values of self-expression dominate. It is proposed to implement behavioural economics mechanisms in state policy of Ukraine while making economic decisions. Conclusions. Due to the new trends in economic development such as informatization, and digitalization, new ways of the economic behaviour of the population take place. One of the key aspects is the psychological approach to the decision-making process in the economic environment. Thus, behavioural economics becomes of great importance in modern society.